Performance improvements

[VorpalBlade]

• Mtime disk cache for Debian (see Debian support #45)
• Disk cache: refresh on access?
• Disk cache - sled: Too many IO threads?
• Tweak heuristics in fs_load_contents
• Look into map types (ahashmap vs btreemap, dashmap vs concurrent-map etc)
  Especially important for building the package ID to package map, which cannot use dashmap due to lack of covariance.
• Files backend builds packages internally :(
• mtree2: what about &mut for smallvec/vec?
• Build performance: Move cli into sub-crates so xtask is cheaper for paketkoll-only builds (for AUR packages this matters)