Backport fix for #73: (protobuf) in 2.21 - Cannot resolve inner types in protoc definitions

[cowtowncoder]

Summary

Backports #656 (fixed on 3.x for 3.1.0) to the 2.21 branch, to fix #73.

TypeResolver._findAndResolve() didn't handle dot-notation references to
nested message types (e.g. t1.i1), so schemas like:

message t1 {
    message i1 {
        optional uint32 x = 1;
        optional uint32 y = 2;
    }
}
message t2 {
    optional t1.i1 z = 1;
}

failed with:

IllegalArgumentException: Unknown protobuf field type 't1.i1' for field 'z' ...

Adds _findDottedType() to resolve such references by navigating into the
outer type's nested elements (recursively, for arbitrary nesting depth).