closes#5201
Currently, if a user sets an external package to have a prefix that is
one of the system paths (like '/usr') the module files that are
generated will prepend '/usr/bin' to 'PATH', etc. This is particularly
nasty at the time when a module file is unloaded, and e.g. paths like
'/usr/bin' will be discarded from PATH.
This PR solves the issue skipping system paths when a prefix inspection
is made to generate module files.