Added support for including .proto files in remote .jars on the --proto_...

...path when compiling with protoc.

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_sources(jar(org='foo.com', name='bar', rev='1.2.3'), jar(org='bar.com', name='foo', rev='3.2.1')),
  ...,
)

An extra ivy step called 'ivy-imports' (rather than ivy-resolve) has been added to accomodate this. It occurs after bootstrap and before codegen.

Note: Not ready for full rbcommons review yet. Still need to test properly (with actual jars of .protos on the nexus somewhere), and there's probably ways (I hope) that ivy_imports can be cleaned up.
I don't want you do to a ton of work only to have pants-devel send us in another direction. I think we should post this to RBCommons as is, offer to take another pass at removing unused code if they agree copying the ivy step is the way we want to go.  If coverage is working, we might be able to use that to help us determine what is dead.