move pkgcore plugins to the pkgcore_checks.plugins namespace

This should help avoid breakage when regenerating the plugin cache on
install. Previously if you had an old version of pkgcore-checks
installed along with newer snakeoil and/or pkgcore then the plugin cache
regen phase during the install process could error out since old
pkgcore_checks code would try to call APIs or do other things that had
been changed in snakeoil and/or pkgcore.

To summarize, we should try to stick with separate plugin namespaces for
each consumer of pkgcore so they don't influence each other when
regen-ing the plugin cache.