Fix wrong transitive snapshot package resolution

The fix enforce a few rules about snapshot dependencies resolution : SNAPSHOT version of packages cannot be installed unwillingly anymore . It means that for a SNAPSHOT version package to be installed you have to: Request it explicitly on the mp-install command line or Allow SNAPSHOT resolution with --snapshot or Have it as a strict dependency (explicit min and max version being SNAPSHOT) of a requested or already installed SNAPSHOT version package Note: When trying to install a release package having a strict SNAPSHOT dependency (which is very bad !), resolution will fail if --snapshot flag is not active