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.
#require docutils gettext
$ $TESTDIR/check-gendoc fr
checking for parse errors