expand docs on package dependency checking in `raco setup`
raco setup: use cache for `pkg-directory` in dep checking
raco setup: continue checking when a package is missing
raco pkg: adjust staging of Git clone installs and updates

If the clone directory's checkout includes a target commit, then
use the clone directory directly for staging (i.e., for checking
dependencies and collisions). That way, changes made locally are
used for metadata checks.