Code for protolib-test artifact pulled in from external repository to demonstrate protobuf-gen 'imports' feature

Followon to https://rbcommons.com/s/twitter/r/592 and https://rbcommons.com/s/twitter/r/720/

Code for protolib-test artifact pulled in from external repository to demonstrate protobuf-gen 'imports' feature

We had this code internally before, now that we have testprojects/maven_layout, there is a good place to put it.

Testing Done:
I deployed the new 1.0.1 version of the protolib-test artifact to nexus.

Manually ran pants goal bundle  examples/src/java/com/pants/examples/protobuf:protobuf-imports-example  and it worked, confirmed the ivy cache now contains the new artifact.

CI passed.

Bugs closed: 521

Reviewed at https://rbcommons.com/s/twitter/r/936/