Support separate download verb to prepopulate caches

[DaanDeMeyer]

We should support a download verb that makes sure all necessary packages and software are downloaded without building the image. This download verb would likely be implemented by having it prepopulate the mkosi cache which is then automatically picked up when we do the actual image build. Optionally, the download verb should take a mkosi manifest and download the specific package versions listed in there. Ideally the download verb also integrates with prepare scripts to allow those to download whatever they need to as well.

[gsegatti]

Curious as to what the implementation would potentially look like, in its simplest form. Would it be basically to run the if not cached portion of build_image(), for whatever distribution specified?

Or Am I way out of my depth here?

[keszybz]

I guess that for dnf, we'd essentially want to do a normal transaction, but break it off after packages have been downloaded. I don't think dnf has an option to do that. There is a system-upgrade download plugin that does that for dnf4. Maybe we should talk with the dnf folks about how to implement this best.

[DaanDeMeyer]

Let's close this in favor of #1112. I'm not sure anymore that a download verb is the way to go