bp2build: add support for cc_object's objs and exclude_srcs properties.

objs contains module references to other cc_objects that will be used
for linking later. This maps to cc_library deps.

Also support exclude_srcs, and added tests.

Test: bp2build generate, sync, bazel build //bionic/...

Change-Id: I21200ff73f24bcf5357d9df8dcb5519cde532a77