feat(build): Support compiling well-known protobuf types (#522)

tests/wellknown-compiled/src/lib.rs

@@ -0,0 +1,9 @@
+pub mod google {
+    pub mod protobuf {
+        tonic::include_proto!("google.protobuf");
+    }
+}
+
+pub fn grok() {
+    let _empty = crate::google::protobuf::Empty {};
+}