Fixes #24295: API export of groups doesn't export the categories as dependencies

[fanf]

https://issues.rudder.io/issues/24295

The chosen new mapping is one that match the LDAP/UI representation of group categories and groups:

archive/
 `- groups
      |- category_with_name
      |     |- category.json // the category info: uuid, name, etc
      |     `- some_group.json // the group as before
      `- some_group_under_root_category.json

This mapping is so-so, since the group serialisation also contains the group category id, so we duplicate the information, and if we move a group, it doesn't actually change its category, which is surprising. But I wanted to have the same json serialisation than the one from API.

I hesitated with a mappring like:

archive/
 |- group-categories.json // one big json will the categories 
 `- groups
      |- some_group.json // the group as before
      `- some_group_under_root_category.json

But it was strang to have a mixe of "one item for each groups" and "one big json with lots of categories".
Still, maybe it would be easier to manipulate.

[fanf]

never do that, it breaks when the thing doesn't exist

[fanf]

we use a ref to avoid building list of list and then flatmapping in the end

[Normation-Quality-Assistant]

This PR is not mergeable to upper versions.
Since it is "Ready for merge" you must merge it by yourself using the following command:
rudder-dev merge https://github.com/Normation/rudder/pull/5521
-- Your faithful QA
Kant merge: "Live your life as though your every act were to become a universal law."
(https://ci.normation.com/jenkins/job/merge-accepted-pr/82359/console)

[fanf]

OK, merging this PR