protos should import one another via globally valid paths

Right now the proto sub-bits depend on each other via jar since they build via maven. In those jars, the proto files themselves are dumped in the root. They should be placed in at least "beam/whatever.proto" or possibly "apache/beam/whatever.proto". This will make the proto files valid in a global namespace (like Java and Go imports and, I dare say, all well thought out import mechanisms)