fix: leave wpcap.dll to the Npcap install

xemu loads wpcap.dll with a plain LoadLibrary (winpcap-loader/loader.c),
and it works only with the Npcap driver, whose installer puts it on the
DLL search path. The schema has no marker for a file a separate system
install provides, so the entry is unsourceable with that reason. The
copy at the BIOS root of RetroBat, RetroDECK and eight other platform
lists, which no search order reaches, leaves them.

emulators/xemu.yml

@@ -295,6 +295,7 @@ files:
 
   - name: wpcap.dll
     required: false
+    unsourceable: "installed by the Npcap driver, which the user installs"
     description: "Npcap packet capture library"
     note: >-
       Loaded by name on Windows, with every entry point resolved individually,

tests/test_launcher_placement.py

@@ -284,8 +284,10 @@ class XboxOnRetroBat(unittest.TestCase):
         at_root = {f["dest"].lower() for f in manifest["files"] if not f.get("root")}
         # The launcher writes the two EEPROMs and the disk image, mounts an
         # .iso with its own tool, and each Cxbx build reads keys.bin from
-        # its own folder.
+        # its own folder. wpcap.dll comes with the Npcap driver.
-        self.assertFalse(at_root & {"eeprom.bin", "xbox_hdd.qcow2", "dokan2.dll", "keys.bin"})
+        self.assertFalse(at_root & {
+            "eeprom.bin", "xbox_hdd.qcow2", "dokan2.dll", "keys.bin", "wpcap.dll",
+        })
         placed = {(f.get("root"), f["dest"]) for f in manifest["files"]}
         self.assertIn(("emulators", "cxbx-reloaded/keys.bin"), placed)
         self.assertIn(("emulators", "chihiro/EEPROM.bin"), placed)