Handle .proto files that end up in a different package

Bazel poses a strict requirement that .proto files and proto_library
must be in the same package. This CL handles this automatically by
creating the proto_library in a separate dir/package if necessary

Implementation details
- Partition the `srcs` by package. `srcs` has been computed using
  `transformSubpackagePath`, so the information about packages is
  available at this point
- Create a proto_library in each package by using
  `CommonAttributes.Dir`. Collect all these additional libraries
  and put them in `info.Proto_libraries` so that they get added as deps
  of (cc|python|...)_proto_library
- Add an import_prefix to the proto_library in subpackages relative to
  the current directory. This relies on the assumption that every src is
  beneath the current directory (Soong will complain if a path in
  Android.bp contains ../)

filegroup module type uses a separate code-path to create proto_library.
This will be handled in the next CL in stack.

Test: bp2build unit tests
Test: TH
Test: Built the failing internal module mentioned in
b/292583584#comment1

Bug: 292583584

Change-Id: I437fc89092321b26c5f0511387cde9e84084d6f9