Only fetch when needed

[SeanTAllen]

When corral fetch is run, it should check to see if the result can be satisfied without doing anything.

For example,

if I have already fetched 1.0.0 of a dependency and have it locally, I shouldn't fetch again if corral fetch is run. It's possible the tag moved. In that case, the user should really do a clean of some sort to update. tags shouldn't move.

If I don't have a version, then it should fetch again to get the latest.