Revert "make anon_package_prefix lazy"

This reverts commit 1c6ed517dc250ca1191b2e3ba0cdf9e8f7129d79.

...which I now remember why I had as non-lazy; this needs to be done early in
the instance lifecycle.
wip! better docs

...any, really
test anon_package_prefix, as well

...for sanity's sake :)