Make top-level rules usable with Bazel's `layering_check`

[coryan]

Bazel enforces "Header inclusion rules": https://bazel.build/reference/be/c-cpp#hdrs

When the layering_check feature is enabled, none of the top-level targets are usable. They have no hdrs value:

https://github.com/googleapis/google-cloud-cpp/blob/8ecdff6cd9ad79e2b2b847edd83205fb8ae65071/BUILD.bazel#L79-L87

• [x] I think we need to change the code to something like:

 [cc_library( 
     name = "{library}".format(library = library), 
     hdrs = ["//google/cloud/{dir}:hdrs".format(dir=google_cloud_cpp_library_dir_name(library)],
     deps = [ 
         "//google/cloud/{library_dir}:google_cloud_cpp_{library}".format( 
             library = library, 
             library_dir = google_cloud_cpp_library_dir_name(library), 
         ), 
     ], 
 ) for library in GA_LIBRARIES + TRANSITION_LIBRARIES] 

• [x] Verify all libraries, specially the hand-crafted libraries, have a filegroup defined for the headers.
• [x] It might be desirable to remove anything with /internal/ in the name from this filegroup too.
• [ ] We should enable layering_check in the package once this is working
• [ ] We should verify the quickstart programs compile with layering_check once this is working

[coryan]

We need to wait until gRPC compiles clean with layering_check enabled before turning this on in our package.

[scotthart]

Still waiting on our dependencies to function with layering_check enabled.

[scotthart]

See if https://github.com/grpc/grpc/issues/34482 being closed has made grpc work with layering_check