Fixed a bug with upstream-removed packages that could skip an uninstall

and then fail to install with a 'multiple versions' error
Accept to (re)install upstream-removed packages if their metadata is still present

the previous check was too strict and made opam-rt fail
Fixed cleanup of metadata
Fixed bug with not finding archives of upstream-removed packages for removal

Closes #1193 (the error was actually a bit different)
Merge pull request #1196 from OCamlPro/fix-multiple-versions-byg

Fixed a bug with upstream-removed packages that could skip an uninstall