feat: file mame device sets by their machines

The -listxml names, for every machine, its parent (romof), the devices of its default configuration (device_ref) and the cards its slots offer (slotoption). A machine takes the system of the first set on its romof chain a written entry lists, and passes it to each device it can instantiate. A device set reached from one system gets system:, from several the new per-entry systems:, from none neither, and keeps serving every system of the profile. The gap report, the core extras and slots read the entry's own systems, so generate_pack --system and the installer pick by them.

tests/test_install_selection.py

@@ -578,13 +578,16 @@ class InstallMatchesASystemPack(unittest.TestCase):
     """`install.py --system X --region R` against a real `generate_pack.py
     --system X --region R` build. The cases are the ones a review reproduced:
     ColecoVision's extras filed at destinations another system declares, and
-    PicoDrive's Mega CD variant group spread over two systems."""
+    PicoDrive's Mega CD variant group spread over two systems. The last pick
+    holds MAME device sets filed under several systems (systems:), under one,
+    and under none."""
 
     PLATFORM = "retroarch"
     CASES = (
         (["coleco-colecovision"], []),
         (["sega-mega-cd"], ["north-america"]),
         (["sony-playstation"], ["japan"]),
+        (["elektronika-bk"], []),
     )
 
     @classmethod