Fix 2337 (and similar) - Ignore existing partitions

[svartkanin]

This fixes #2337, #2316, #2324, #2304 and #2317

• The formatting validation is only applicable to partitions that should be created or modified
• Creating BTRFS volumes is only applicable if the partition should be created or modified

[codefiles]

My proposed fix in https://github.com/archlinux/archinstall/issues/1971#issuecomment-1666673560:

Filtering the partitions to be validated to only partitions with a status of modify (ModificationStatus.Modify) or create (ModificationStatus.Create) should fix this unless there is a purpose behind validating partitions with a status of existing (ModificationStatus.Exist) or delete (ModificationStatus.Delete).

You credited my investigative work from this comment in #2031 but maybe missed the proposed fix. Thanks for taking care of this.

[svartkanin]

I missed that conversation there somehow, thanks for the analysis and the confirmation that this is the right path