This changes the behavior of the hl_cache to the old one.
- when the capture exists as a hlgroup -> use it
- when hl_map contains a mapping -> use it
- else do nothing (before: map capture to non-existing capture)
Before also captures `@foo.bar` would intend to use the hlgroup `foo.bar`
which results in a confusing error since hlgroups can't contain dots.
f489d98653
