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.
