Better error message when given an invalid package target.
Renaming types: Spec -> Target
Treat deprecated packages as if they are not in the core.

This is what cpanm does.  Although I'm not sure it is the
correct thing to do.
Just a TODO comment.
get_distribution() wasn't returning the right thing.

When looking for a package among all distributions in the
repo, we were only returning the latest one.  Now, we return
the latest one that satisfies the target.  This is the same
behavior that the Locator has.
Update change log