feat: let an archive join a set

Kronos reads kronos/stvbios.zip, else stvbios.zip at the root of the
system directory, and a ST-V game stops without either
(libretro.c:1608-1617, 1676-1681 at the pin). The archive under
archive_prefix and the root copy are one need, but only loose entries
could be members, so the required-only pack carried both.

An archive's own entry now joins a set like a loose file. verify
--emulator and the emulator pack settle archive units with the loose
entries, the required-only pick covers them, find_undeclared_files
and cross_reference record them, and the unit under archive_prefix
ranks before the same name at the root. Its members still settle the
archive by their contents. Kronos' stvbios.zip is grouped stv-bios.

No other profile reads one archive under two paths: odin and
tgpulse_next_m1 place their own entries at the prefix path, and
flycast's naomi.zip has one path per build.

scripts/alternatives.py

@@ -197,7 +197,12 @@ def core_file_spares(
     and verify alike.
     """
     answered = {id(e) for e in entries if e.get("set_declared")}
-    rest = [e for e in entries if id(e) not in answered]
+    # An archive under the core's own folder is read before the same name
+    # at the root (Kronos), so archive units rank first among equals.
+    rest = sorted(
+        (e for e in entries if id(e) not in answered),
+        key=lambda e: not e.get("archive"),
+    )
     return answered | required_only_spares(
         rest,
         owner=lambda e: e.get("source_profile") or e.get("profile") or "",