fix protobuf "import" symbol names

i factored out the logic of loading & parsing a protobuf file into a List[Node], and use that to parse imported protobuf files. they're digested into a map of (symbol name -> metadata), where metadata is the package name, and whether it's an enum or a message.

in the process, i broke (and removed) loading files from resources. i don't think it was ever fully exposed, but it won't work with importing files from the path.

the generator then does a pass that turns symbol names into fully-qualified package+symbol names, if they are in the map of imported symbols.