[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