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:

• 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] 

• Verify all libraries, specially the hand-crafted libraries, have a filegroup defined for the headers.
• 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