Better handling of package removed from upstream
Fixed 'opam install' when some packages have been removed from upstream
Fixed upgrade when packages have been removed from upstream
The solver needs to know about those packages to be able to remove them, which was not the case ;
there is still a glitch if you try eg to reinstall 'a' when 'b' depends on 'a': nothing will be
done about it, since 'b' can't be recompiled. If 'c' depends on 'b', it won't be recompiled either.

But since there is no clear way of handling that situation (the only correct thing would be to remove b and c ; that would need some more work),
it seems reasonable for now to ask for an 'opam upgrade' in such pathological cases.

Closes: #740
Merge pull request #788 from AltGr/fix-no-longer-available

Fix behaviour when packages have been removed from upstream