Currently `xkb_keymap_num_layouts` may return a greater number than the
number of layouts configured using RMLVO, because we allow symbols
sections to define various groups per key.

This is unintuitive and kind of buggy: groups should be added via rules
by setting an explicit `:n` modifier.

Fix: when parsing a keymap using RMLVO resolution:
- Get the expected layouts count from the resulting KcCGST.
- Drop the groups after the first one in included symbols sections. This
  will ensure that a symbol section can only define one group per key.

Notes:
- Compiling a keymap string directly is unaffected.
- RMLVO resolution may still produce more groups than the input layouts.
  Indeed, some legacy rules in xkeyboard-config rely on this to insert
  automatically a US layout before the given non-Latin one, resulting
  in two layouts while only one was given.