policy: add helper to import cext/pure module These functions are sysstr API since __import__() and getattr() hate byte strings on Python 3. There's a minor BC, which is ImportError will be raised if invalid HGMODULEPOLICY is specified. I think this is more desired behavior. We're planning to add strict checking for C API compatibility. This patch includes the stub for it.

demandimport: strictly check missing locals argument locals may be an empty dict.