fix: copy standalone files from their pack path

The registry names one path per standalone copy for every platform,
but each manifest places the file its own way, so the installer
looked where the pack had put nothing and copied nothing: EmuDeck's
aes_keys.txt for Azahar, Batocera's switch/prod.keys. Each manifest
copy now names its source, which the installer validates and reads.

install.py

@@ -1396,6 +1396,10 @@ def _validate_manifest(data: object, plat: str) -> dict:
             _safe_relative_path(
                 entry["file"], f"standalone_copies[{index}].file"
             )
+        if "source" in entry:
+            _safe_relative_path(
+                entry["source"], f"standalone_copies[{index}].source"
+            )
         if "pattern" in entry:
             pattern = entry["pattern"]
             if (
@@ -2432,7 +2436,10 @@ def do_standalone_copies(
                 if fnmatch(f.name, entry["pattern"]) and f.is_file()
             ]
         else:
-            src = bios_path / entry["file"]
+            # `source` is where this platform's manifest put the file; `file`
+            # names it as the registry does for every platform, and laid out
+            # that way it was missing wherever the pack places it elsewhere.
+            src = _destination_path(bios_path, entry.get("source") or entry["file"])
             sources = [src] if src.exists() else []
 
         if not sources: