This CL adds support for cc_library to correctly split c, as and cpp
srcs in shared/static nested props, as well as splitting
the *filegroup* deps in those props, where each filegroup is expanded
into its own c, cpp and as srcs filegroups. This ensures that the
correct sources go into cc_library_static's underlying cc_libraries for
c, cpp and as sources respectively.
See the bp2build conversion test for a better visualization.
Bug: 183064430
Test: TH
Change-Id: I29add5140672d042adff65527d8b65f4a5f0a05b
14a8bda31e