_pkg_use_enabled: return frozenset (531112)

Since commit 9ba4f2aa6a2755a73fb652b4557919047d649fd1, _pkg_use_enabled
needs to consistently return a frozenset, so that the result is
hashable.

Fixes: 9ba4f2aa6a27 ("_slot_operator_update_probe: memoize use_reduce (529660)")
X-Gentoo-Bug: 531112
X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=531112
Acked-by: Brian Dolbec <dolsen@gentoo.org>