rules: Resolve relative include statements using XKB paths
Contrary to keymap files, the `! include` statement in rules does not
lookup include paths added to `xkb_context`. So it is not possible e.g.
to import another file in the same folder without using an absolute path.
- Added path utils: `is_absolute(path)`.
- Added XKB paths lookup to enable e.g. `! include evdev` to work.
- Added test.
