A pinned package should always have the same version number.

This means:
(i) if the package is already installed, it should be the version of the installed package
(ii) otherwise abitrary pick the latest version

Previously, we didn't check (i) which was causing weird "Multiple versions installed for package ..." errors when a new version of the pinned package appears upstream.

This should help fixing bugs similar to #541 (not sure it really fixes #541, need more investigation)