fix: place standalone files in the linked folder

standalone_path is relative to the directories a standalone build
resolves it against, now declared per profile (standalone_directories).
A launcher record that links or passes a whole folder of the BIOS tree
under one of them gives that folder, and platform_layouts rebases the
standalone view once for the builder, the manifest, verify and slots
(launchers.standalone_root, common.standalone_destination).

Flycast on RetroPie and ROCKNIX reads bios/dc, on EmuDeck
bios/flycast/bios, on Batocera (FLYCAST_BIOS_PATH) and RetroBat
(Dreamcast.BiosPath) bios/dc. The ROCKNIX, Batocera and RetroBat
scrapers now record the patched data directory, the environment
variable and the setting that say so.

schemas/emulator.schema.json

@@ -886,6 +886,13 @@
       "type": "string",
       "description": "Branch the port was read from, when it is not the default one."
     },
+    "standalone_directories": {
+      "type": "array",
+      "items": {
+        "type": "string"
+      },
+      "description": "What the standalone build resolves standalone_path against, as its code names it: a directory ($XDG_DATA_HOME/flycast), a setting (Dreamcast.BiosPath), an environment variable or the function returning the directory. A platform launcher record that links or passes a folder of its BIOS tree under one of these puts the standalone files in that folder."
+    },
     "prose_written_at": {
       "type": "string",
       "pattern": "^[0-9a-f]{40}$",