Allow again removal of package/{old,}stable entries from *-needed list

Since the regular expression was tightened to fix a bug and not remove
e.g. spice ans spice-gtk from a *-needed.list removal of specific
entries of packages/stable or packages/oldstable got broken (wich is
used by the Debian security team to mark entries which only need an
update in one of the supported suites).

Retain the desired fixed behaviour but try to allow to properly remove
package/{old,}stable entries again.

Fixes: b3070631 ("bin/gen-DSA: Fix package removal from the needed_file. Don't remove packages starting with the same string as the to be removed package.")
Signed-off-by: Salvatore Bonaccorso <carnil@debian.org>