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/nodes/equalfuncs.c

@@ -31,7 +31,7 @@
 
 #include "miscadmin.h"
 #include "nodes/extensible.h"
-#include "nodes/relation.h"
+#include "nodes/pathnodes.h"
 #include "utils/datum.h"
 
 
@@ -814,7 +814,7 @@ _equalOnConflictExpr(const OnConflictExpr *a, const OnConflictExpr *b)
 }
 
 /*
- * Stuff from relation.h
+ * Stuff from pathnodes.h
  */
 
 static bool