PackageManager: Store packages hierarchically, by version ...

As explained in the changelog, this hierarchy makes much more sense
than the current one, and will make it possible to read the version
from the directory name.

add $DPATH environment variable for file locations (#2281) ...

* error out on unknown environment variables

* add env + config for package + settings locations

fix #229

* adjust to review, extend $DPATH test

* review round 2