Add some mechanism to renumber existing tracks according to per_disc_numbering setting

version: 1.3.13

I have per_disc_numbering set to no in my config to change any songs I import from tracks per disc to tracks per album (as detailed in the documentation).

Most of the time this works. However, when importing an album that _is_ _not_ on MusicBrainz, the only way to bypass MB and import the album is to select "Use As-Is". When this option is selected, the tags are completely untouched. This includes the track numbering scheme, so per_disc_numbering is essentially ignored.

Is there/can we add a way to import music without using MusicBrainz while still letting beets' tagging plugins work?

Start by tracing the import path for the "Use As-Is" option and how the per_disc_numbering setting is applied during normal MusicBrainz imports. Compare both paths and define a way for imports without MusicBrainz to still run tagging plugins, with track numbers changed from per-disc to per-album when configured.

Written by the indexing model from the issue text.