I have collected code that used to be in lib/adminlib.php, lib/accesslib.php and user/selector/lib.php into a new admin/roles/lib.php file.
And I added standard GPL and Moodle comments to all the files in admin/roles.
This implements the page showing the table. It is not yet integrated into the tab bar. To try this, go to an Assign roles page (one with URL .../admin/roles/assign.php?contextid=...) and change the 'assign' to 'explain'.