RFC: fix to detect mounted LUKS-encrypted btrfs subvolumes

[intentionally-left-nil]

• This fix issue: Could not detect root at mountpoint /mnt/archinstall #2194

PR Description:

This is a prototype fix for detecting already-mounted btrfs volumes, when inside of a LUKS encrypted partition. Without this fix, the code only detects the LUKS partition and not its mounted counterpart.

This code adds the detection for this case, and then recurses down to the children to find the btrfs subvolumes. The reason this is a prototype (request for comment) is that I don't know what to do about the partition information. The subvolume is technically part of the LUKS partition, but e.g. the path is /dev/mapper/ not /dev/sda1 (which points to LUKS). etc.

This PR does seem? to work when running archinstall on my own setup, but I don't have enough knowledge of the codebase to understand what the correct solution is.

I'm happy to carry this PR forward, with guidance. Otherwise I'm happy to turn it over to @Torxed or others to implement a more robust solution

Tests and Checks

• I have tested the code!
  

[intentionally-left-nil]

The PR isn't quite right, now btrfs.progs isn't being installed. The attempted commit I pushed to fix this just made things worse...

[Torxed]

I just wanted to swing by with a comment so you know I've seen it and will take a look :)

[intentionally-left-nil]

Thanks!

I think there's a few pieces to get right:

• The UUID (especially when creating any boot entries)
• The filesystem type (btrfs) so that you end up with btrfs.progs installed
• The root subvolume id

and there are probably others that don't pertain to my specific scenario