get rid of catalog.uniload

There is no longer a uniload catalog when running from a release, since
the release contains whole isopackets (programs) not
isopacks (packages).

We only need a uniload-specific catalog when we're actually rebuilding
isopackets (not in order to load them), so we now have it as a local
variable in the two places that build isopackets.

The deleted code in package-loader.js was specific to the prebuilt
uniload package which no longer exists.