Ease ability to depend on external workspace from parent workspace

[ittaiz]

Essentially I'd like to expose an API like addExternalRepository(repoName) which copies the repo directory from runfiles and adds it to the test workspace (as a local repository).
Part of the challenge is that it seems that at least for some use-cases you need to manually generate BUILD files and those need to have custom content (for example use java_import) so I'm not sure how this should work.
I do know the current API is hard to use.
Some very rough parts can be observed in addExternalRepositoryFor, repositoryDeclarationFor, writeWorkspaceFileWithRepositories from https://github.com/bazelbuild/bazel-integration-testing/blob/21425e04588cb687da085b15524a337bfb28c6c6/javatests/build/bazel/tests/integration/BazelBaseTestCaseTest.java

[ittaiz]

I think #71 solved this for archives.
We should probably solve it for git_repository and we can close this one