These name collisions should be avoided with attributes, all sorts of
issues can arise from those. We already warned in the attributes
(but not the vertex groups) list if those were found.
Previously, creating a vertex group with the same name as an already
existing attribute would allow this (and give said warning), and creating
an attribute with the same name as an already existing vertex group
would silently fail (as in: not return a layer) -- and then due to an oversight
in 101d04f41f (which assumed a valid layer would always be returned
by `BKE_id_attribute_new`) would even crash.
Now name collisions between vertex groups and attributes are avoided,
unique names will be found across attributes and vertex groups if either
`BKE_id_attribute_calc_unique_name` or `BKE_object_defgroup_unique_name`
is called.
This is done by unifying the checks and callbacks for both into a single.
Pull Request: https://projects.blender.org/blender/blender/pulls/109910
12ef20990b