feat: place core files per handed system dir

A platform can hand each libretro core a system directory of its own
(core_system_dirs). platform_layouts then gives one view per directory,
so the gap report, the builder, the manifest, slots and cross_reference
place a core's files below each folder it is handed, and a core handed
none gets no file. Resolution strips the folder again and searches the
collection by the core's own path.

scripts/cross_reference.py

@@ -41,6 +41,7 @@ from common import (
     require_yaml,
     resolve_local_file,
     platform_layouts,
+    system_dir_path,
     standalone_set,
     select_emulator_profiles,
 )
@@ -652,7 +653,7 @@ def slot_key(f: dict, standalone: bool = False) -> tuple:
     return (
         fname,
         f.get("archive"),
-        where or fname,
+        system_dir_path(f, where or fname),
         f.get("system"),
         f.get("variant_group"),
         f.get("mode", "both"),
@@ -727,7 +728,7 @@ def _cross_reference_profile(
         if is_standalone:
             effective_path = f.get("standalone_path") or f.get("path") or fname
         else:
-            effective_path = f.get("path") or fname
+            effective_path = system_dir_path(f, f.get("path") or fname)
         slot = slot_key(f, is_standalone)
         seen_key = slot
         if len(layouts) > 1: