Improve Bzlmod toolchain registration

Before this commit, users had to do the following in their
`MODULE.bazel` file to register a custom Go SDK:

```
go_sdk = use_extension("@rules_go//go:extensions.bzl", "go_sdk")
go_sdk.download(name = "my_go", version = "1.19.5")
use_repo(go_sdk, "my_go")
register_toolchains("@my_go_toolchains//:all")
```

This has several drawbacks:
* Names can collide with SDKs defined by transitive dependencies as they
  all live in the same namespace.
* The SDK has to be brought into scope with `use_repo`.
* Since the toolchain definitions are separate from the SDK repo, users
  need to know the naming convention for the separate toolchain repo for
  their `register_toolchains` calls.
* It encourages using the SDK repo directly, which is almost never
  correct: Toolchain resolution should be used instead, possibly in the
  form of the new convenience target `@rules_go//go`.

With this commit, Bazel toolchain definitions are emitted into a single
repo and registered by rules_go itself. SDK repo names are mangled and
kept as an implementation detail of rules_go. Module dependencies are no
longer allowed to register SDKs with a non-mangled name, but an
exception is made for the root module for the sake of backwards
compatibility and to address unforeseen use cases.

As a result of this change, SDK registration now looks as follows:

```
go_sdk = use_extension("@rules_go//go:extensions.bzl", "go_sdk")
go_sdk.download(version = "1.19.5")
```

.bazelci/presubmit.yml

@@ -31,7 +31,7 @@ tasks:
       - "--allow_yanked_versions=all"
     build_targets:
       - "//..."
-      - "@go_sdk//..."
+      - "@go_default_sdk//..."
     test_targets:
       - "//..."
   macos:

MODULE.bazel

@@ -22,9 +22,9 @@ go_sdk.download(
     name = "go_default_sdk",
     version = "1.18.3",
 )
-use_repo(go_sdk, "go_default_sdk_toolchains")
+use_repo(go_sdk, "go_toolchains")
 
-register_toolchains("@go_default_sdk_toolchains//:all")
+register_toolchains("@go_toolchains//:all")
 
 bazel_dep(name = "gazelle", version = "0.27.0")
 

go/private/extensions.bzl

@@ -1,9 +1,9 @@
-load("//go/private:sdk.bzl", "go_download_sdk", "go_host_sdk")
+load("//go/private:sdk.bzl", "go_download_sdk_rule", "go_host_sdk_rule", "go_multiple_toolchains")
 load("//go/private:repositories.bzl", "go_rules_dependencies")
 
 _download_tag = tag_class(
     attrs = {
-        "name": attr.string(mandatory = True),
+        "name": attr.string(),
         "goos": attr.string(),
         "goarch": attr.string(),
         "sdks": attr.string_list_dict(),
@@ -15,40 +15,130 @@ _download_tag = tag_class(
 
 _host_tag = tag_class(
     attrs = {
-        "name": attr.string(mandatory = True),
+        "name": attr.string(),
         "version": attr.string(),
     },
 )
 
+# This limit can be increased essentially arbitrarily, but doing so will cause a rebuild of all
+# targets using any of these toolchains due to the changed repository name.
+_MAX_NUM_TOOLCHAINS = 9999
+_TOOLCHAIN_INDEX_PAD_LENGTH = len(str(_MAX_NUM_TOOLCHAINS))
+
 def _go_sdk_impl(ctx):
+    multiple_version_module = {}
+    for module in ctx.modules:
+        if module.name in multiple_version_module:
+            multiple_version_module[module.name] = True
+        else:
+            multiple_version_module[module.name] = False
+
+    toolchains = []
     for module in ctx.modules:
-        for download_tag in module.tags.download:
+        for index, download_tag in enumerate(module.tags.download):
             # SDKs without an explicit version are fetched even when not selected by toolchain
             # resolution. This is acceptable if brought in by the root module, but transitive
             # dependencies should not slow down the build in this way.
             if not module.is_root and not download_tag.version:
                 fail("go_sdk.download: version must be specified in non-root module " + module.name)
-            go_download_sdk(
-                name = download_tag.name,
+
+            # SDKs with an explicit name are at risk of colliding with those from other modules.
+            # This is acceptable if brought in by the root module as the user is responsible for any
+            # conflicts that arise. rules_go itself provides "go_default_sdk", which is used by
+            # Gazelle to bootstrap itself.
+            # TODO: Investigate whether Gazelle can use the first user-defined SDK instead to
+            #  prevent unnecessary downloads.
+            if (not module.is_root and not module.name == "rules_go") and download_tag.name:
+                fail("go_sdk.download: name must not be specified in non-root module " + module.name)
+
+            name = download_tag.name or _default_go_sdk_name(
+                module = module,
+                multi_version = multiple_version_module[module.name],
+                tag_type = "download",
+                index = index,
+            )
+            go_download_sdk_rule(
+                name = name,
                 goos = download_tag.goos,
                 goarch = download_tag.goarch,
                 sdks = download_tag.sdks,
                 urls = download_tag.urls,
                 version = download_tag.version,
-                register_toolchains = False,
             )
-        for host_tag in module.tags.host:
+
+            toolchains.append(struct(
+                goos = download_tag.goos,
+                goarch = download_tag.goarch,
+                sdk_repo = name,
+                sdk_type = "remote",
+                sdk_version = download_tag.version,
+            ))
+
+        for index, host_tag in enumerate(module.tags.host):
             # Dependencies can rely on rules_go providing a default remote SDK. They can also
             # configure a specific version of the SDK to use. However, they should not add a
             # dependency on the host's Go SDK.
             if not module.is_root:
                 fail("go_sdk.host: cannot be used in non-root module " + module.name)
-            go_host_sdk(
-                name = host_tag.name,
+
+            name = host_tag.name or _default_go_sdk_name(
+                module = module,
+                multi_version = multiple_version_module[module.name],
+                tag_type = "host",
+                index = index,
+            )
+            go_host_sdk_rule(
+                name = name,
                 version = host_tag.version,
-                register_toolchains = False,
             )
 
+            toolchains.append(struct(
+                goos = "",
+                goarch = "",
+                sdk_repo = name,
+                sdk_type = "host",
+                sdk_version = host_tag.version,
+            ))
+
+    if len(toolchains) > _MAX_NUM_TOOLCHAINS:
+        fail("more than {} go_sdk tags are not supported".format(_MAX_NUM_TOOLCHAINS))
+
+    # Toolchains in a BUILD file are registered in the order given by name, not in the order they
+    # are declared:
+    # https://cs.opensource.google/bazel/bazel/+/master:src/main/java/com/google/devtools/build/lib/packages/Package.java;drc=8e41dce65b97a3d466d6b1e65005abc52a07b90b;l=156
+    # We pad with an index that lexicographically sorts in the same order as if these toolchains
+    # were registered using register_toolchains in their MODULE.bazel files.
+    go_multiple_toolchains(
+        name = "go_toolchains",
+        prefixes = [
+            _toolchain_prefix(index, toolchain.sdk_repo)
+            for index, toolchain in enumerate(toolchains)
+        ],
+        geese = [toolchain.goos for toolchain in toolchains],
+        goarchs = [toolchain.goarch for toolchain in toolchains],
+        sdk_repos = [toolchain.sdk_repo for toolchain in toolchains],
+        sdk_types = [toolchain.sdk_type for toolchain in toolchains],
+        sdk_versions = [toolchain.sdk_version for toolchain in toolchains],
+    )
+
+def _default_go_sdk_name(*, module, multi_version, tag_type, index):
+    # Keep the version out of the repository name if possible to prevent unnecessary rebuilds when
+    # it changes.
+    return "{name}_{version}_{tag_type}_{index}".format(
+        name = module.name,
+        version = module.version if multi_version else "",
+        tag_type = tag_type,
+        index = index,
+    )
+
+def _toolchain_prefix(index, name):
+    return "_{}_{}_".format(_left_pad_zero(index, _TOOLCHAIN_INDEX_PAD_LENGTH), name)
+
+def _left_pad_zero(index, length):
+    if index < 0:
+        fail("index must be non-negative")
+    return ("0" * length + str(index))[-length:]
+
 go_sdk = module_extension(
     implementation = _go_sdk_impl,
     tag_classes = {