manifest: split manifestdict into high-level and low-level logic The low-level logic type (_lazymanifest) matches the behavior of the C implementation introduced in a5f1bccd. A future patch will use that when available.

manifest: do parsing inside manifestdict contstructor This shape to the code will make using a C implementation of the manifest storage easier.

manifest: move parsing functions up in file These functions are about to change signature and be hidden inside the manifestdict constructor. Doing the code motion now as an isolated change to make things easier to review.

dispatch: offer suggestions of similar-named commands When suggestions are available, we show those suggestions instead of showing some help output.