Fix cache inconsistency in RemotePackageSource

When a package using getPackageById on the RemotePackageSource, the package found is added alone to the PackageListCache even if it is empty. This leads to cache inconsistency because further calls to listPackages methods will think the cache is not empty and return only the added package.
We should add a timestamp on each package entry in addition of the timestamp on the list entry and so be able to tell that the whole list is not up to date even if it contains some packages (see Julien Carsique comments).

Due to this bug, you might have a "Package not found" exception when trying to install a package along with an already installed snapshot package.
A workaround is to run the same command again (as the previously installed snapshot package is now uninstalled)