Use tarred repositories (or something) to make SourceManager testing fast, easy, and controlled

[sdboyer]

archive/tar exists. Rather than cloning repositories from remotes - which, even in the simple case, adds up to being quite slow - it'd be quite a bit better to have repositories stored within the vsolver repo that we can untar into place as needed for tests, rather than having to repeat expensive clone ops over and over again.

...maybe. Or maybe it's just a test harness we really need for the manager side of testing. Not really clear right now.

[fabulous-gopher]

This issue was moved to golang/dep#443