Rename files and headers related to index AM

The following renaming is done so as source files related to index
access methods are more consistent with table access methods (the
original names used for index AMs ware too generic, and could be
confused as including features related to table AMs):
- amapi.h -> indexam.h.
- amapi.c -> indexamapi.c.  Here we have an equivalent with
backend/access/table/tableamapi.c.
- amvalidate.c -> indexamvalidate.c.
- amvalidate.h -> indexamvalidate.h.
- genam.c -> indexgenam.c.
- genam.h -> indexgenam.h.

This has been discussed during the development of v12 when table AM was
worked on, but the renaming never happened.

Author: Michael Paquier
Reviewed-by: Fabien Coelho, Julien Rouhaud
Discussion: https://postgr.es/m/20191223053434.GF34339@paquier.xyz

src/include/access/amvalidate.h

@@ -1,36 +0,0 @@
-/*-------------------------------------------------------------------------
- *
- * amvalidate.h
- *	  Support routines for index access methods' amvalidate functions.
- *
- * Copyright (c) 2016-2019, PostgreSQL Global Development Group
- *
- * src/include/access/amvalidate.h
- *
- *-------------------------------------------------------------------------
- */
-#ifndef AMVALIDATE_H
-#define AMVALIDATE_H
-
-#include "utils/catcache.h"
-
-
-/* Struct returned (in a list) by identify_opfamily_groups() */
-typedef struct OpFamilyOpFuncGroup
-{
-	Oid			lefttype;		/* amoplefttype/amproclefttype */
-	Oid			righttype;		/* amoprighttype/amprocrighttype */
-	uint64		operatorset;	/* bitmask of operators with these types */
-	uint64		functionset;	/* bitmask of support funcs with these types */
-} OpFamilyOpFuncGroup;
-
-
-/* Functions in access/index/amvalidate.c */
-extern List *identify_opfamily_groups(CatCList *oprlist, CatCList *proclist);
-extern bool check_amproc_signature(Oid funcid, Oid restype, bool exact,
-								   int minargs, int maxargs,...);
-extern bool check_amop_signature(Oid opno, Oid restype,
-								 Oid lefttype, Oid righttype);
-extern bool opfamily_can_sort_type(Oid opfamilyoid, Oid datatypeoid);
-
-#endif							/* AMVALIDATE_H */