Removing link-time cross-module refs in contrib

Pursuant to Andres' suggestion in
https://www.postgresql.org/message-id/20161002223927.57xns3arkdg4hu6x@alap3.anarazel.de
attached is a draft patch that gets rid of link-time references
from hstore_plpython to both hstore and plpython. I've verified
that this allows "LOAD 'hstore_plpython'" to succeed in a fresh
session without having loaded the prerequisite modules first.

The patch seems pretty successful in terms of being noninvasive to
the code. I think the major objection to it would be that we no
longer have any direct compiler-verified connection between the
signatures of the called functions in hstore/plpython and what
hstore_plpython thinks they are. That is, if someone were to
modify hstore and change the signature of say hstoreCheckKeyLen,
this would not cause any compiler complaints in hstore_plpython,
just runtime problems.

A slightly ugly way of alleviating that risk would be to put the
function typedefs right beside the externs in the originating
modules, eg in hstore.h

extern size_t hstoreCheckKeyLen(size_t len);
+typedef size_t (*hstoreCheckKeyLen_t) (size_t len);

I'm not sure if that is a net improvement or not --- thoughts?

If we were to push forward with this idea, the remaining work
would be to fix the other two contrib transform modules similarly,
after which I would want to revert the changes in commit cac765820
and later that suppressed linker errors for unresolved symbols in
contrib links. The possibility of getting back that error detection
is actually the main motivation for this IMO.

Comments?

regards, tom lane