switch import: fix import of non-repo pinned packages, and be more tolerant

Will now only display a warning and ignore unavailable packages in the import
Allow pinning a directory without specifying name if it's in a local opam file

This makes the first case of #1430 doable in a single command ('opam pin .')
More explicit messages at unpin
Merge pull request #1452 from OCamlPro/pin-and-switch

Fixed import of pinned packages with the new features, and improved pin command