Add test for bug #460304.
Fix bug #460304.
Don't use splitext for .ebuild/.tbz2 check.
Use endswith(tuple).
_add_dep: remove questionable need_restart logic

This need_restart logic was apparently intended to interact with
want_restart_for_use_change, but doesn't appear to be necessary
anymore. We really don't want the need_restart flag to prevent
"missing dependency" backtracking.