[FEATURE] Split configuration sources include non-split default (Revision 2)

This change changes the behavior of split configuration sources to
include the default non-prefixed source as a fallback for increased
flexibility, allowing some packages to have split sources and others
to have a single source.

Also a check in the has method is made more strict.

Change-Id: I60c1b40c291b184d22f290c1ffcd02e43fe493a8
Related: #37854
Releases: master, 2.2
Revision-Id: fb2fc011092f3c0c5bb69a2942a1cfe9f192e60e