Merge pull request #823 from AltGr/fix-aspcud-and-pin

Fix aspcud and pin
Do not delete compiler descriptions of still installed compilers at update
Only complain about missing metadata for packages in the current switch
Accepting to version-pin uninstalled packages
Reclassing errors about removed stale files as warnings
Fixed warnings about missing metadata on pinned packages
The overlays are now used in state.opams which should be more consistent, and
warnings about metadata missing in a repo can be avoided with an overlay

AMEND (comments on #825): don't store overlays in the state cache (that could get inconsistent).
Instead, honor overlays everywhere ('available', universe...)
Remove the dev-pkg directory at uninstall, avoiding warning on next command
Atm we re-download anyway (really need to cache upstream files), so this seems harmless

AMEND (comments on #825): remove dev archives only when package isn't installed in any switch anymore
Fixed error when a non-current compiler has been removed