Fix: make sure backup_dir exists
Metadata update for all dev packages

not only the pinned ones
Display causes for reinstall commands too

The solver wasn't called at all for this simpler case, so the cause computation
step was bypassed.
Fixed opam printer for "%{ident}%"

the simplification doesn't work in all cases
Merge pull request #1305 from OCamlPro/dev-opam-fixes

Closes #1304 (both issues)