(maint) Don't stub Pathname.new when using the autoloader

Stubbing out Pathname.new when working with objects that may autoload
can demonstrate bad behavior, where autoloading will entirely fail to
occur. It seems that there is sufficient state stored in autoloading
that the one failure cascades into many other tests involving
autoloading.

This commit removes the use of stubs in the module install face in favor
of using actual paths; by doing this we can avoid stubbing most of the
system and just apply expecations to the data being passed around.