Allow to override drupal_system_listing() directory precedence in settings.php (D7 only)

(This is a Drupal 7 issue only. Drupal 8 has already fixed this problem in other ways.)

Currently in D7, drupal_system_listing() works with a more or less hardcoded list of directories, in the $searchdir variable. (A poorly named variable, btw)
Elements at the end of $searchdir have precedence. E.g. a module in ./sites/default/modules has precedence over a module in ./modules.

One thing that is currently missing is the scanning of the base profile.
Other projects may have other needs.

Unfortunately, changing this behavior in D7 would be a BC break.
Exposing a hook to alter the scanning is not a good idea either, because drupal_system_listing() is not supposed to depend on modules.

What's left is sites/*/settings.php.
(Or I hope so. drupal_system_listing() always comes after settings.php, or not? (*))
We could expose a setting that would provide a custom function to provide the base directories to scan in, such as "sites/all/".
There could even be an existing function that does include the base profile, and that only needs to be activated in settings.php.

As a preparation step, we could factor out two functions from drupal_system_listing():

• _drupal_system_listing_directories($directory) would give us the directories for the $searchdir variable, such as "sites/all/modules".
• _drupal_system_listing_base_directories() would give us the *base* directories, such as "sites/all/".
  (Yes this should be with trailing slash, so that we don't have to treat Drupal root (empty string) as a special case.)

An override from settings.php would provide an alternative implementation of _drupal_system_listing_base_directories().

(*) If it turns out that settings.php does not work (e.g. during install), we could let the active profile do it instead. E.g. a flag in the profile info file.