support for nested definitions in .proto files

[acg]

Currently, the python lwpb code can't resolve nested definitions like this:

 message A {
   message B {
     enum C {
     ...
     };
   };
 };

 message D {
   optional A.B.C stuff = 1;
 };

The rules for resolving type names follow C++ scoping.

This feels like an unnecessary complication in the protocol buffers spec -- I've personally never had a need for it and simply use a flat list of definitions in my .proto files.

However, it's in the spec, so see if it can be implemented in the descriptor code.