Support PEP 517 list based backend-path
    
    The PEP 517 shows an example backend-path like this:
    
        [build-system]
        # Defined by PEP 518:
        requires = ["flit"]
        # Defined by this PEP:
        build-backend = "local_backend"
        backend-path = ["backend"]
    
    https://www.python.org/dev/peps/pep-0517/#source-trees
    
    See that backend-path is a list. Our code previously only supported string path.
    
    Obviously a string path is wrong, but we keep it to support projects that have
    made the mistake, such as flit-core.
    
    Add a small integration test for both cases.
    Note that the new spec files deliberately don't do much, to save CI time.