Improve proto rules

[coreywoodfield]

I made the same changes as #217, #211, and #289, without realizing that it had already been done (and pulled the test from #217, thanks @bjchambers). Since merge checks are failing for two of those and none of them seem very active, I figured I'd give it a shot as well. Resolves #218. Tests pass locally.

Overview of changes:

• Stop passing dependencies as sources to protoc. Previously, generating scala code for one.proto (in tests/proto) would also generate code for zero.proto, since one depends on zero. Since we're already also generating scala code for zero.proto by itself, the scala code for zero.proto gets generated twice, and each generated scala jar contains the scala code for all of its proto dependencies. With this change, generating scala code for one.proto doesn't generate code for zero.proto. Rather, the generated scala library for one has a dependency on the generated scala library for zero.
• Pass proto_path arguments to protoc. Since we're no longer passing dependencies as sources to protoc, we pass the root directories of all dependencies to protoc so that it can find the proto files we depend on itself. This also allows third-party protos to be used (as mentioned in Add transitive path arguments when compiling protobuf #289) since protoc knows about all the directories where proto files are actually found, rather than just the directories where the protos being compiled presently are found.

[SrodriguezO]

LGTM, thanks @coreywoodfield. CI looked good save for format; pushed up fix. Will merge once CI finishes. Travis is borked.. so here's the link.

Also thanks (and apologies for this falling from our radar) @bjchambers, @kerinin, and @tjarvstrand