Read from all state files we can find

Not just the first writable one we find. This (for example) allows a (root) install of
perl6+panda into /usr/local; along with some modules such as URI and
MIME::Base64. A user can then run 'panda install LWP::Simple' and will
not need to reinstall URI and MIME::Base64, because panda will now see
the modules installed in /usr/local
Don't read extra statefiles if doing site install

This could cause problems if there were modules installed in
/root/.perl6 for example. If we're doing a site install, all
dependents need to be in site as well.
Merge pull request #76 from retupmoca/multistate

Read from all state files we can find