move(algebra/hom/*): Move group hom files together (#12647)
Move
* `algebra.group.freiman` to `algebra.hom.freiman`
* `algebra.group.hom` to `algebra.hom.basic`
* `algebra.group.hom_instances` to `algebra.hom.instances`
* `algebra.group.units_hom` to `algebra.hom.units`
* `algebra.group_action_hom` to `algebra.hom.group_action`
* `algebra.iterate_hom` to `algebra.hom.iterate`
* `algebra.non_unital_alg_hom` to `algebra.hom.non_unital_alg`
