toolchain docs: toolchain_type attributes should use absolute label

[laszlocsomor]

Description of the problem / feature request:

The toolchain docs give an example where the toolchain rule references the toolchain_type by relative label:

toolchain(
    name = "barc_linux_toolchain",
    exec_compatible_with = [
        "@bazel_tools//platforms:linux",
        "@bazel_tools//platforms:x86_64",
    ],
    target_compatible_with = [
        "@bazel_tools//platforms:linux",
        "@bazel_tools//platforms:x86_64",
    ],
    toolchain = ":barc_linux",
    toolchain_type = ":toolchain_type",
)

https://docs.bazel.build/versions/master/toolchains.html

This is wrong -- even though it's syntactically correct, toolchain resolution will not work if this rule is ever used from an external repository.

Instead, the toolchain_type should always reference absolute labels, even if the target is in the same package:

toolchain(
   ...
   toolchain_type = "@repo_name//path/to/package:toolchain_type",
)

Have you found anything relevant by searching the web?

Related bug: #6828

[mfarrugi]

This is super difficult to debug, would love to see this immediately error out.

[dkelmer]

/cc @dkelmer

[katre]

This is a special case of #11128.

[sventiffe]

@laszlocsomor @juliexxia this one hasn't been updated since a while (the referenced issue mentioned on May 11 is still open though). Is this issue still relevant?

[laszlocsomor]

@sventiffe Thanks for asking!

It seems to work now. Here's a complete example built on top of the tutorial.