convert: Multiple automatic conversions

Could we add a feature whereby we select what happens at various stages of the import process when it relates to auto converting?

I want my canonical reference library to be flac. So if I get music that's lossless but not flac, on import, I want it to convert to flac /first/, and then import. That's all well and dandy, and it does it perfectly this way.

But I -also- want beets to manage my mobile media, so /after/ the flac has been imported and tagged and cover-arted and all the magical whiz I want beets to do, I would then like it to /also/ convert the canonical reference to opus.

I know I can do this manually, but it's a bit of a pain in the bum.

I envision something like pre_import_format, post_import_formats (or similar). Then it can just use the same convert.formats that are already there.

Start at the import process and trace how the existing convert.formats configuration is applied. Define the pre-import and post-import conversion stages, then verify that lossless non-FLAC files become FLAC before import and imported FLAC files are also converted to Opus afterward.

Written by the indexing model from the issue text.