For instance, if the SDK repository is `@go_sdk`, the toolchain rules will be defined in `@go_sdk_toolchains`. This avoids eagerly fetching `@go_sdk`, fixing bazelbuild#3196.

One caveat: `:sdk_version_setting` needs to be defined in the same repo as the toolchain rule, e.g. in `@go_sdk_toolchains`. I tried `target_settings = "@go_sdk//:sdk_version_setting"`, but that causes `@go_sdk` to be eagerly fetched again.

This means that `@go_sdk_toolchains` needs to know the SDK version. There are two possibilities:
* The user specifies the version up front, for instance via `go_register_toolchains(version = ...)`. In that case, `@go_sdk_toolchains` uses this version.
* The user does not specify the version up front. `go_wrap_sdk`, `go_download_sdk`, and `go_local_sdk` do not require a `version` attribute. In this case, we have no choice but to inspect the SDK itself to obtain the version, forcing an eager load. However, we can at least add an optional `version` attribute to these rules, so that users can avoid the eager fetch if they wish.

In all cases, when a `version` attribute is provided by the user, the implementation (lazily) validates that it matches the actual SDK version.