core: Fix app paths used in running Zotonic when starting from a release

When Zotonic is started from a release it may happen that ebin folder is actually in an .ez archive (as generated by reltool). In that case the path of an ebin module (as read by set_path/get_path in zotonic_app) will be different than the path to the priv folder. For example:

`code:lib_dir for priv:"/usr/home/g/somepath/rel/lib/zotonic-0.10-dev/priv"`
`Zotonic app path:"   /usr/home/g/somepath/rel/lib/zotonic-0.10-dev.ez/zotonic-0.10-dev"`

This changes ensures that priv is read from a normal folder (default for priv when using reltool) even if Zotonic itself is started from an .ez archive.