Adds a proto_library rule for each well known type proto:

$ bazel query "filter(\".*_proto$\", \"...\")"
//:wrappers_proto
//:timestamp_proto
//:struct_proto
//:field_mask_proto
//:empty_proto
//:duration_proto
//:compiler_plugin_proto
//:descriptor_proto
//:api_proto
//:type_proto
//:source_context_proto
//:any_proto

Bazel users can reference these proto_library rules for their own
language specific rules i.e.

java_proto_library(
  name = "any_java_proto",
  deps = ["@com_google_protobuf//:any_proto"],
)

Also set the workspace name to "com_google_protobuf", as proto_library
rules reference protobuf that way.