Prior to this commit, we were trying to be far too smart when updating
a package's package-local-nicknames: we'd only call
`remove-package-local-nickname` on the set difference of the old
nicknames and the new nicknames; that is, we would not remove and then
reinstall a package-local-nickname which was already present and we
wanted to keep. The bug which caused #51 was that the set-difference
considered only the nickname, not the package to which it
referred. But rather than worrying about accurately computing the
minimal set of nicknames to remove, this commit just has
`ensure-package` remove all package-local-nicknames, then reinstall
the ones it wants. This may be either a performance improvement or
regression depending on whether computing set differences is more or
less expensive than manipulating package data structures, but my
suspicion is that no one cares either way.

Also, during the course of my debugging, I [Phoebe Goldman] added a few line-comments
to the definition of `ensure-package` so that I could tell what
various parts of its body were doing. I'd rather not write those
comments again, so I'm including them in this commit.

Tests incorporated and changelog updated.