Corral should support a bundle alias mechanism

[cquinn]

To allow local development of Pony bundles, and replacement of public bundles with local or private shared versions. It would be convenient if this aliasing could be a simple override mechanism to define aliases for bundles referenced in deps that are otherwise unmodified. The aliases should apply to transitive deps too. Something like:

{
  "deps": [{ "locator": "github.com/ponylang/corral-test-repo.git/bundle2" }],
  "aliases: [{ "locator": "github.com/ponylang/corral-test-repo.git/bundle2", "alias":  "../bundle2" }]
}

But it could also be an alias field in each dep, or both. And, with either approach, Corral could load them from a distinct file that could be kept local for local development.