Use upstream gps lock diff implementation

[carolynvs]

From #256, I implemented lock diffing and thought it may be more appropriate to have it in the gps package. At the very least it would remove some workarounds I had to put in because of unexported fields. Though one option is to update gps to export those fields and keep the diffing in dep.

• dep.LockDiff struct, dep.LockedProjectDiff struct, dep.StringDiff struct. These are the structures which hold the diff and are printed to the console during a dry-run.
• dep.diffLocks(), dep.diffProjects(). The diff implementation.
• dep.getVersionInfo(): Decomposes a gps.Version interface into the underlying metadata that is persisted in the lock file (revision, branch and version).

[carolynvs]

I've submitted a PR to gps moving LockDiff and co. Since there has been a bit of drift in the gps API and I have open PR which touches LockDiff (the TOML annotations), I'll hold off on submitting a PR to complete the move until that settles down.

[sdboyer]

Renaming and repurposing this issue as a reminder to remove what we have locally once we pull in the latest gps.