'Build a working set from a requirement spec. Rewrites sys.path.'

@classmethod
def _build_from_requirements(cls, req_spec):

    ws = cls([])
    reqs = parse_requirements(req_spec)
    dists = ws.resolve(reqs, Environment())
    for dist in dists:
        ws.add(dist)
    for entry in sys.path:
        if (entry not in ws.entries):
            ws.add_entry(entry)
    sys.path[:] = ws.entries
    return ws
