Auto merge of #3040 - matklad:rootless-readlockfile, r=alexcrichton

Auto merge of #3040 - matklad:rootless-readlockfile, r=alexcrichton

Don't use current package when reading lockfile

Looks like this `default` thing is basically a hack, which is fired when the lockfile references some package which is no longer present. That is, if I remove `default` and just `unwrap` source id, the only test which fails is `git_with_lockfile` because it has a typo in the lockfile.

So I've just changed the `default` to some dummy path. Perhaps a more correct solution would be to ignore such packages completely when loading lockfile.