cc Bp2build support for genrules that generate .proto file

If `srcs` contains a gensrcs/genrule module, the current bp2build module
will put it in the catch-all `srcs` attribute. This is reserved for .cpp
sources, so if the genrule produces a .proto/.aidl/... file, this will
fail.

This handles genrules that produce .proto files. To implement this, this
creates an additional partition that detects if the other module is a
genrule/gensrc that produces .proto files. If true, it will append it to
the proto partition.

This CL does not handle
- genrule that produce .c/.aidl/.yacc/.... files. They will continue to
  be partitioned into the catch-all partition
- java modules

Test: unit tests
Test: TH
Bug: 293205700
Change-Id: Ib720fdf3a053ff5dd256e6faa632e3fa7776966d