Convert contrib/hstore_plpython to not use direct linking to other modules.

Previously, on most platforms, we allowed hstore_plpython's references
to hstore and plpython to be unresolved symbols at link time, trusting
the dynamic linker to resolve them when the module is loaded.  This
has a number of problems, the worst being that the dynamic linker
does not know where the references come from and can do nothing but
fail if those other modules haven't been loaded.  We've more or less
gotten away with that for the limited use-case of datatype transform
modules, but even there, it requires some awkward hacks, most recently
commit 83c24920.

Instead, let's not treat these references as linker-resolvable at all,
but use function pointers that are manually filled in by the module's
_PG_init function.  There are few enough contact points that this
doesn't seem unmaintainable, at least for these use-cases.  (Note that
the same technique wouldn't work at all for decoupling from libpython
itself, but fortunately that's just a standard shared library and can
be linked to normally.)

This is an initial patch that just converts hstore_plpython.  If the
buildfarm doesn't find any fatal problems, I'll work on the other
transform modules soon.

Tom Lane, per an idea of Andres Freund's.

Discussion: <2652.1475512158@sss.pgh.pa.us>