Extra debug logging during plugin loading

As a new-ish puppet user, it is incredibly easy to screw up your
module path (e.g., by adding a directory that directly contains
module/plugin contents to the module path, rather than adding the
*parent* directory. This basically means that there would be a
'lib' directory directly in your module path.)

This commit just adds a few debug messages that may give a user
hints about what could be wrong with their module path when
we are able to detect these sorts of situations.