Running pytest with "--pyargs" will result in Items with empty "parent.nodeid" if run from a different root directory

[cboelsen]

When running pytest with "--pyargs" from a directory that isn't a parent directory of the packages under test, then the Items created will have an empty parent.nodeid (i.e. "::test_function").

This is because the _makeid() method in FSCollector compares the path of the Module to the rootdir:

relpath = self.fspath.relto(self.config.rootdir)