Fix import of groups without path

[mlocati]

If we import this XML:

<?xml version="1.0"?>
<concrete5-cif version="1.0">
    <groups>
        <group name="Test" />
    </groups>
</concrete5-cif>

The group Test will always be created, even if it already exists.

Let's fix this.

[mlocati]

I fixed another bug too: now the package is correctly assigned to the newly created groups