Reorganize partitioning code

Reorganize partitioning code

There's been a massive addition of partitioning code in PostgreSQL 11,
with little oversight on its placement, resulting in a
catalog/partition.c with poorly defined boundaries and responsibilities.
This commit tries to set a couple of distinct modules to separate things
a little bit.  There are no code changes here, only code movement.

There are three new files:
  src/backend/utils/cache/partcache.c
  src/include/partitioning/partdefs.h
  src/include/utils/partcache.h

The previous arrangement of #including catalog/partition.h almost
everywhere is no more.

Authors: Amit Langote and Álvaro Herrera
Discussion: https://postgr.es/m/98e8d509-790a-128c-be7f-e48a5b2d8d97@lab.ntt.co.jp
	https://postgr.es/m/11aa0c50-316b-18bb-722d-c23814f39059@lab.ntt.co.jp
	https://postgr.es/m/143ed9a4-6038-76d4-9a55-502035815e68@lab.ntt.co.jp
	https://postgr.es/m/20180413193503.nynq7bnmgh6vs5vm@alvherre.pgsql