Add a @gapic_third_party indirection to the gapic build.

[pmuetschard]

Makes overriding of where the gapic dependencies come from a bit easier.

[pmuetschard]

Let me know if this helps with the maven deps.

[Qining]

hmm, I don't know if this will be helpful. Generally what I have to do is manually downloading all the dependencies, then create local_repository for them. As all the dependencies in GAPID are wrapped with maybe_repository, those already existing dependencies won't be pulled again. This is also the approach we have always been using.

In this way, for most dependencies, the BUILD file and WORKSPACE file will also have their local copies, so the package structure won't change.

[pmuetschard]

Well, the point of this change is that you no longer have to do that! You now have lots of options:

1. Create one extra repository called "gapic_third_party" that contains the 6 third party targets we need (:grpc, :okhttp, :lwjgl, :guava, :jface and :swt).
2. Check the jars into your main repo and then in the WORKSPACE file call the gapic_third_party function with the mappings pointing to them and it will generate the gapic_third_party repo BUILD file for you.
3. With the mappings you can have the jars almost anywhere.
4. ...

[Qining]

Great, thanks, this definitely helps a lot.