PackageCache: Remove on-disk cache and soft reload

On-disk cache will be replaced by a new class.

Soft refresh was an optimization that wasn't actually very optimized in
0.9.0; it used to only have to look at an in-memory WatchSet and check
those files, but in 0.9.0 it also re-read the whole isopack. Will add
this optimization back in if necessary and safe later.