[SP3] Improve evaluating the old repositories (bsc#1185822)

[lslezak]

The Problem

• https://bugzilla.suse.com/show_bug.cgi?id=1185822
• During migration usually the new repositories have a different URL so they point to new package versions (e.g. the Basesystem Module for SLE15-SP2 is replaced with SLE15-SP3 repository with different name and URL)
• During migration YaST offers to remove the old repositories because the old repositories/packages could potentially cause conflicts or other dependency problems
• But the detection of the old repositories does not work properly if the new repository is the same as the old one (X replaced by X), in that case the repository looks like as if it was old and should not be used anymore

The Solution

• The improved code loads the current (SP3) registered repositories and these repositories are excluded from the old ones

Testing

• Tested manually, see below

Screenshots

• The original behavior: the "new" SP3 repositories with the NVidia module are preselected for removal
  

• The improved behavior: only the old SP2 repository is listed
  

[coveralls]

Coverage increased (+0.05%) to 39.084% when pulling 3bd87e7 on old_addons into 532f643 on SLE-15-SP3.

[lslezak]

Submitted to SP3 manually in https://build.suse.de/request/show/242399