Remove confusing 'nothing to do' message at uninstall

when there is a .install file
Fix removal of unpinned user-local packages

their overlay was cleaned up too early, losing their uninstall info.
We now only cleanup the overlay at removal, or at load time if there
is other metadata available (from the repo) or the package is not installed.
Don't install metadata of pinned packages to <opam>/packages
Cleanup empty directories in <opam>/packages
Merge pull request #1464 from OCamlPro/unpin-remove-fix

Fixed issues with pinned metadata handling and uninstall