Check `$this` is instance of Config in get()

Check that `$this` is instance of `Concrete5_Model_Config` in
`Config::get()`

Because `Config::get()` is not declared static but is called statically,
it will inherit `$this` from the calling class.

If that class happens to have a `$pkg` property that is an object, we
will actually call `getPackageID()` on that object