Fix more qmk generate-api fallout from userspace support

After qmk#22222 `list_keymaps(fullpath=True)` returns absolute paths which
might point to the userspace directory tree; however, the implementation
of `qmk generate-api` expected to get paths relative to `qmk_firmware`.
The problem was partially fixed in qmk#22618 for the generated `url` value;
however, the `path` value for keymaps was still incorrect (this also
made the subsequent code overwrite the `keymap.json` files in the
working copy instead of writing those files converted to plain JSON into
the API output directory).

Fix the `path` value for keymaps to contain the keymap path relative to
`qmk_firmware` as it was before the userspace changes, and skip any
userspace keymaps which might have been found.

lib/python/qmk/cli/generate/api.py

@@ -128,12 +128,15 @@ def generate_api(cli):
         # Populate the list of JSON keymaps
         for keymap in list_keymaps(keyboard_name, c=False, fullpath=True):
             keymap_rel = qmk.path.under_qmk_firmware(keymap)
+            if keymap_rel is None:
+                cli.log.debug('Skipping keymap %s (not in qmk_firmware)', keymap)
+                continue
             kb_json['keymaps'][keymap.name] = {
                 # TODO: deprecate 'url' as consumer needs to know its potentially hjson
                 'url': f'https://raw.githubusercontent.com/qmk/qmk_firmware/master/{keymap_rel}/keymap.json',
 
                 # Instead consumer should grab from API and not repo directly
-                'path': (keymap / 'keymap.json').as_posix(),
+                'path': (keymap_rel / 'keymap.json').as_posix(),
             }
 
         keyboard_dir.mkdir(parents=True, exist_ok=True)