Add bp2build support for cc_library_static.

Also refactor bp2build-related code for cc_library_headers.

Test: Added unit test.

Test: bp2build-sync.py write; bazel build //bionic/... still works (but bp2build is disabled for most cc_library_static targets for other reasons)

Change-Id: I37c856be20a47b154909338a22a7dba1ab55693f