Revert "Rename files and headers related to index AM"

This follows multiple complains from Peter Geoghegan, Andres Freund and
Alvaro Herrera that this issue ought to be dug more before actually
happening, if it happens.

Discussion: https://postgr.es/m/20191226144606.GA5659@alvherre.pgsql

src/backend/optimizer/path/costsize.c

@@ -73,8 +73,8 @@
 
 #include <math.h>
 
+#include "access/amapi.h"
 #include "access/htup_details.h"
-#include "access/indexam.h"
 #include "access/tsmapi.h"
 #include "executor/executor.h"
 #include "executor/nodeHash.h"
@@ -539,7 +539,7 @@ cost_index(IndexPath *path, PlannerInfo *root, double loop_count,
 	 * for scanning the index, as well as the selectivity of the index (ie,
 	 * the fraction of main-table tuples we will have to retrieve) and its
 	 * correlation to the main-table tuple order.  We need a cast here because
-	 * pathnodes.h uses a weak function type to avoid including indexam.h.
+	 * pathnodes.h uses a weak function type to avoid including amapi.h.
 	 */
 	amcostestimate = (amcostestimate_function) index->amcostestimate;
 	amcostestimate(root, path, loop_count,

src/backend/optimizer/plan/planner.c

@@ -18,8 +18,8 @@
 #include <limits.h>
 #include <math.h>
 
+#include "access/genam.h"
 #include "access/htup_details.h"
-#include "access/indexgenam.h"
 #include "access/parallel.h"
 #include "access/sysattr.h"
 #include "access/table.h"

src/backend/optimizer/util/plancat.c

@@ -17,8 +17,8 @@
 
 #include <math.h>
 
+#include "access/genam.h"
 #include "access/htup_details.h"
-#include "access/indexgenam.h"
 #include "access/nbtree.h"
 #include "access/sysattr.h"
 #include "access/table.h"