Draft: unmkinitramfs: Stop splitting into "early" and "main" subdirectories

We originally unpacked each cpio archive into its own subdirectory for security reasons. Since we are now able to stitch together multiple cpio archives to feed to a single cpio process, that concern no longer applies.

We maintained the split up to the release of "trixie" out of concern for possible breakage of callers that relied on this split. Since that release is done, stop splitting and always unpack everything directly into the output directory.

(This goes on top of !168 (merged) and shouldn't be applied yet.)