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

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