Still use a $module_folder relative path

Previously, `wordpoints_get_modules()` returned the list of modules
with the basename module file keys of the array calculated relative the
optional `$module_file` parameter. This is the same behavior as
`get_plugins()`.

This was unconsciously changed to be relative to the `$module_root` in
6913767. This reverts it back.

See #104
Remove a dead assignment in wordpoints_get_modules

See #104