feat: give each set its requirement and one pick

Three changes to the search-list model, one commit because the readers
and the profiles must move together for the schema to hold.

Set requirement in a groups map. pom2 falls back on apple2.rom for the
][ and the ][+, and on apple2e.rom for the //e it boots and for the
unenhanced //e: one required on the file could not say both. A profile
now declares groups: {name: {required}}, a member writes no required
of its own (schema and validate_schemas refuse one) and load_emulator_
profiles derives it, true when one of its sets is. group takes a name,
a list, or a map from set to rank where the rank differs per set. pom2
is grouped from SystemProfile.cpp and MainWindow.cpp:246-296 at its pin.

One member per required set under --required-only. The pick is the
first the code reads among the held members the region filter leaves,
by rank then declaration order; a set the platform declares a member
of is carried by that declaration. The emulator pack, verify
--emulator, the platform pack, its manifest and verify --platform read
alternatives.required_only_spares or core_file_spares.

variant_group is now selection_slot: files a core chooses between by
a rule other than first found (disc territory for PicoDrive and
Pizza Boy SC, the o2em_bios option, a WebAssembly probe in SWF
Player). Where the code takes any one, the field became a group:
ARMSX1 (ArmSxBackend.java:149-157, name order), ePSXe (load_bios
0x433540-0x433621 of the 2.0.5 Linux x64 binary, one configured path,
HLE when BiosHLE is set), FPse and FPse-NG, DXX-Rebirth's HOGs
(inferno.cpp:472-512). odin and zesarux carried it with no effect.
slot.py compares ranks only between candidates that share a set.

scripts/common.py

@@ -22,6 +22,8 @@ import zipfile
 from pathlib import Path
 from typing import Any, TextIO
 
+from alternatives import derive_member_requirements
+
 # Every build script imports this module. verify.py, slots.py, validate_pr.py
 # and cross_reference.py started under 3.8 and stopped halfway on a
 # str.removeprefix AttributeError, which named no version.
@@ -2346,6 +2348,7 @@ def load_emulator_profiles(
         if "emulator" not in profile:
             continue
         _normalize_hash_fields(profile)
+        derive_member_requirements(profile)
         profiles[f.stem] = profile
     _emulator_profiles_cache[cache_key] = profiles
     return profiles