Group class not resolved relatively on the group document

Step to reproduce:

• Create a sub-wiki
• Call org.xwiki.contrib.authentication.internal.DefaultUserManager#addToGroup from the sub-wiki with a group on the main wiki.

Expected:
As the group is on the main wiki the Group class is resolved on the main wiki

Currently:

The group class is resolved on the sub-wiki, so we end with an error like this:

WARN  c.x.x.d.XWikiDocument          - Calling xclass [subwiki:XWiki.XWikiGroups] on document [xwiki:XWiki.XWikiAdminGroup]: the wiki part of the xclass reference is wrong, replacing it.