Rename nodes/relation.h to nodes/pathnodes.h.

The old name of this file was never a very good indication of what it
was for.  Now that there's also access/relation.h, we have a potential
confusion hazard as well, so let's rename it to something more apropos.
Per discussion, "pathnodes.h" is reasonable, since a good fraction of
the file is Path node definitions.

While at it, tweak a couple of other headers that were gratuitously
importing relation.h into modules that don't need it.

Discussion: https://postgr.es/m/7719.1548688728@sss.pgh.pa.us

src/backend/optimizer/path/costsize.c

@@ -540,7 +540,7 @@ 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
-	 * relation.h uses a weak function type to avoid including amapi.h.
+	 * pathnodes.h uses a weak function type to avoid including amapi.h.
 	 */
 	amcostestimate = (amcostestimate_function) index->amcostestimate;
 	amcostestimate(root, path, loop_count,
@@ -4072,7 +4072,7 @@ get_restriction_qual_cost(PlannerInfo *root, RelOptInfo *baserel,
  *	sjinfo: SpecialJoinInfo relevant to this join
  *	restrictlist: join quals
  * Output parameters:
- *	*semifactors is filled in (see relation.h for field definitions)
+ *	*semifactors is filled in (see pathnodes.h for field definitions)
  */
 void
 compute_semi_anti_join_factors(PlannerInfo *root,

src/backend/optimizer/util/predtest.c

@@ -21,7 +21,7 @@
 #include "miscadmin.h"
 #include "nodes/makefuncs.h"
 #include "nodes/nodeFuncs.h"
-#include "nodes/relation.h"
+#include "nodes/pathnodes.h"
 #include "optimizer/optimizer.h"
 #include "utils/array.h"
 #include "utils/inval.h"