gh-36889: more libgap in permutation groups ; get rid of _libgap_init_
Moving forward from gap pexpect interface to libgap interface :
- using more libgap in the permutation groups code
- get rid of the barely used `_libgap_init_` method in favor of the
unique `_gap_init_` method.
### :memo: Checklist
- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [x] I have created tests covering the changes.
URL: https://github.com/sagemath/sage/pull/36889
Reported by: Frédéric Chapoton
Reviewer(s): Dima Pasechnik, Frédéric Chapoton
