rustpkg fetching remote packages is broken

[catamorphism]

A recent change of mine broke extern mod foo = github.com/foo/bar/blat;, because now util::find_and_install_dependencies (correctly) tries to find what workspace the package is in before installing it, but (incorrectly) gives up if it's not found locally. The code should be refactored so that the git fetch step can happen before installing.

This regressed because, unfortunately, there are no automated rustpkg tests that fetch a package from a remote repository (since network connectivity was unreliable on the bots).

[alexcrichton]

Would it be possible to have these tests on the bots, but ignore network failures? Ideally we'd do something like raise on the test::skip condition or something like that to indicate that the test should be skipped (and provide different output than if it passed), but it would be awesome to at least test this functionality most of the time.

[catamorphism]

The problem is that because of #9001, there isn't a simple way to determine what unhandled condition a sub-task failed with. Maybe someone else has a bright idea...

[catamorphism]

For testing, @cmr suggested running a local git server on each of the bots, which seems possible, though a bit of a pain in the neck since we'd have to run it separately on each.