Adds deps for imports in generated code

[melindalu]

After golang/protobuf#785 go_grpc_library fails with

$ bazel build //services/example/protos:example_grpc_go_library
INFO: Analyzed target //services/example/protos:example_grpc_go_library (5 packages loaded, 186 targets configured).
INFO: Found 1 target...
ERROR: /Users/melinda/code/example/services/example/protos/BUILD.bazel:8:1: GoCompilePkg services/example/protos/darwin_amd64_stripped/example_grpc_go_library%/github.com/melindalu/example/services/example/protos.a failed (Exit 1)
compilepkg: missing strict dependencies:
	/private/var/tmp/_bazel_melinda/0181ab8bd8f9c21018c34bfa3a8cf24f/execroot/com_github_melindalu_example/bazel-out/darwin-fastbuild/bin/services/example/protos/example_grpc_go_library_pb/services/example/protos/example.pb.go: import of "google.golang.org/grpc/codes"
	/private/var/tmp/_bazel_melinda/0181ab8bd8f9c21018c34bfa3a8cf24f/execroot/com_github_melindalu_example/bazel-out/darwin-fastbuild/bin/services/example/protos/example_grpc_go_library_pb/services/example/protos/example.pb.go: import of "google.golang.org/grpc/status"
No dependencies were provided.