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

[Jun Wu]

Excerpts from Yuya Nishihara's message of 2017-05-09 23:09:07 +0900:
> The auto-rebuild could be HGMODULEPOLICY=compile if we want.

I actually prefer "allow" by default to auto build. People wanting
up-to-date binaries should run `make local` manually.

> > 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 
> 
> I'm okay for the explicit versioning. I can adjust the last patch for that.
> 
> I don't have strong preference since managing API versions would be as
> simple as managing explicit version constants unless we have internal
> dependency in C layer.

We do have complex cases: module-level exception (mpatch.mpatchError), and
parsers.c with complex Python objects. Think about if someone wants to catch
mpatchError, or change the return value of lmiter_iterkeysnext in manifest.c
for example.