This is done by way of a new 'imports' parameter added to the java_protobuf_library target, which accepts jar dependencies as inputs. The syntax for this is:

java_protobuf_library(name='foo',
  imports=[
    jar(org='foo.com', name='bar', rev='1.2.3'), jar(org='bar.com', name='foo', rev='3.2.1'),
    '3rdparty:some-protobuf-jar-library',
  ],
  ...,
)

An extra ivy step called 'ivy-imports' (rather than ivy-resolve) has been added to accomodate this. It occurs after bootstrap and before codegen.

Testing Done:
Added junit test which uses .proto which includes imports .proto files I jarred and stuck on the maven central nexus. ci.sh is green on my machine and on travis. Also tested internally in the square repo.

The proto files on the maven central nexus were bundled with maven and do not reside in the main pants repo.

Bugs Closed: 297 http://github.com/pantsbuild/pants/issues#issue/297

Reviewd at: https://rbcommons.com/s/twitter/r/592/