[bazel] cc_grpc_library incompatible with proto_library that has strip_import_prefix

[jhance]

I believe this likely has the same root cause as #28199.

When I attempt to use cc_grpc_library with a proto_library that has a strip_import_prefix, there is some sort of conflict where protoc attempts to compile both the original proto path and the proto path from within virtual imports (which I assume is an effect of having a strip_import_prefix).

I tried making a few combinations of tweaks to the protoc rules but when I managed to make it not pick up the non-virtual-import proto file, the output was then generated to the incorrect location as well, so I am unsure what the fix would be. (Most of this proto virtual import stuff is very mysterious to me)