[PATCH 12 of 12 RFC] policy: add helper to import cext/pure module

[Jun Wu]

Excerpts from Augie Fackler's message of 2017-05-08 15:22:41 -0400:
> I've taken patches 1-10 since they seem to clean up a lot of things. I
> don't have strong feelings on patch 12, and patch 11 looks fine to me,
> but I didn't take either one.

Patch 11 looks good to me too. It's solving a similar problem with the auto
rebuild patch [1]. If we take patch 11, then the auto rebuild patch becomes
optional, although still nice to have for people wanting to test C code (for
reasons like perf, correctness, etc) explicitly.

Patch 12 is where I disagree. _dualmod still requires careful renaming and
could cause subtle problems that does not get warned by any code checking
tools. They also require extra effort in writing and reviewing related code.
I'd like to go through the explicit versioning way so even the renaming
churn could be avoided.

[1]: https://www.mercurial-scm.org/pipermail/mercurial-devel/2017-May/097329.html