bp2build support for cc_prebuilt_library

while cc_prebuilt_library_(shared|static) were already supported,
the raw cc_prebuilt_library module type was not supported by
bp2build. This CL provides that functionality.

Also did a little bit of refactoring and cleaning.

Bug: 225938765
Test: Confirm output BUILD.bazel files look as expected. Confirm
      that build succeeds.
Change-Id: Ic004f88febc0e5b527f2ef420dbd96ecc55d2e09