Fix detection logic for api.env.in_tree
    
    The logic to detect in-tree builds was broken and ipatests/conftest.py
    had hard-coded in_tree=True.
    
    IPA now considers an environment as in-tree when the parent directory of
    the ``ipalib`` package contains ``ipasetup.py.in``. This file is only
    present in source and never installed.
    
    API bootstrap() does not use ```self.site_packages in site.getsitepackages()``
    because the function call can be expensive and would require path
    normalization, too. The function is also missing from venv site module.
    
    Fixes: https://pagure.io/freeipa/issue/8312
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>