[Bazel] Add bzl_library targets

[RyanDraves]

Add some bzl_library targets for .bzl files. This enables stardoc to do its thing and is helpful in its own right for validating rule & macro dependencies.

Notes:

• Open question: should this PR add bzl_library targets for all .bzl files? I added the ones I personally depend on, but adding it for more files requires bumping the rules_cc version; might be poking the bear?
• I haven't filed an issue yet, will do as desired

[kilograham]

cc @armandomontanez

[armandomontanez]

Bumping rules_cc should be fine.

I should probably take a pass at some of the .bzl libraries and re-assess their intended visibility. For example, configurable_feature was an implementation detail to make it easier to write/maintain the toolchains. I don't think it should be considered part of the public interface.

Btw, thanks for kicking this off, stardoc in the BCR is very cool.