opam-admin: add a '--resolve' option that builds just the archives you need for a specific installation
Merge pull request #1031 from AltGr/opam-admin-resolve

opam-admin: added '--resolve'
Defined empty_universe

This may enable users of opam_lib not to break if we add fields
Merge pull request #1033 from AltGr/empty-universe

Defined empty_universe