This is additional work for the long-term solution to #748.

When an importer tries to constrain a dependency, but for some reason the constraint isn't valid, e.g. the branch was deleted, the tag removed, currently the importer stops with an error. The user must then edit the imported config file to remove the offending data that is triggering dep to import a constraint.

Instead of immediately stopping, the importers should print a warning that it can't preserve the existing constraint. It should still lock to a revision, and infer a sane constraint based on that revision.