Plugins/Packages: ability to overwrite recommended flag per package

This is #106 rebased to current master:

I had the requirement to override the global recommended flag for single packages.

The patch introduce a possibility to handle it. It adds an recommended flag for the Package-Tag. The recommended flag currently does not pass to the packages automatically added as dependencies by the packages plugin itself, but it could be overwritten for these packages as well by adding a Packages-Tag for it. If no recommended flag is given the global value is used.

Note: This removes also the regeneration of the package cache after changing the global value, because it always saves the recommended dependencies separate.