Provide `DefaultInfo` on Go toolchain rules (#4373)

**What type of PR is this?**

Feature

**What does this PR do? Why is it needed?**

This makes it easier to deal with these toolchains in language-agnostic
aspects and helpers (in particular, bazel_env.bzl), that propagate to
toolchain targets.

Even though genrules now support `toolchain_type`s in their
`toolchains`, this does *not* allow Go SDKs to be used in genrules as
they don't expose the path to the Go binary via a Make variable. This is
intentional as we don't want to allow running `go` in `genrule`s.

**Which issues(s) does this PR fix?**

**Other notes for review**