WIP: emit a module per proto file, to support dependencies between them

I ran into #28 trying to use this package, and started trying to fix it.

I think the solution is to generate a module per proto file, which is all this change does so far. For my use case, I think this will be enough to get me up and running.

To handle packages, we can either

1. Ignore them, and just remember which proto file each fully-qualified name came from, so we can resolve package names to module names.
1. Create an outer module for each package, with its proto files as sub-modules.

(1) seems simpler, and is close to what the python back-end does, since it just translates each .proto to a .py, and python files are modules.