Significantly improved suboverlay discovery.

Suboverlays are now discovered and visited according to the customary
conventions of a depth-first search (DFS). Scripts are now sourced on immediate
discovery, permitting functionality defined in the main codebase to be reliably
called by @{compile} during overlay discovery. This is fairly awesome, as it
implies we may now call system-specific testers during compilation itself
(e.g., :is_package_manager_portage()).