pgsql: 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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/fa2cf164aaf91e074be653c28e035f65d80eb666

Modified Files
--------------
contrib/bloom/bloom.h                         |  2 +-
contrib/postgres_fdw/postgres_fdw.h           |  2 +-
src/backend/executor/execAmi.c                |  3 ++-
src/backend/executor/nodeCustom.c             |  1 +
src/backend/nodes/README                      |  2 +-
src/backend/nodes/copyfuncs.c                 |  4 ++--
src/backend/nodes/equalfuncs.c                |  4 ++--
src/backend/nodes/nodeFuncs.c                 |  2 +-
src/backend/nodes/outfuncs.c                  |  4 ++--
src/backend/nodes/print.c                     |  2 +-
src/backend/optimizer/path/costsize.c         |  4 ++--
src/backend/optimizer/util/predtest.c         |  2 +-
src/backend/rewrite/rewriteManip.c            |  2 +-
src/backend/statistics/dependencies.c         |  2 +-
src/backend/statistics/extended_stats.c       |  1 -
src/include/access/tsmapi.h                   |  2 +-
src/include/executor/executor.h               |  2 +-
src/include/executor/nodeCustom.h             |  1 -
src/include/foreign/fdwapi.h                  |  2 +-
src/include/nodes/extensible.h                |  2 +-
src/include/nodes/nodes.h                     |  8 ++++----
src/include/nodes/{relation.h => pathnodes.h} | 12 ++++++------
src/include/optimizer/appendinfo.h            |  2 +-
src/include/optimizer/clauses.h               |  2 +-
src/include/optimizer/cost.h                  |  2 +-
src/include/optimizer/geqo.h                  |  2 +-
src/include/optimizer/inherit.h               |  2 +-
src/include/optimizer/joininfo.h              |  2 +-
src/include/optimizer/optimizer.h             |  4 ++--
src/include/optimizer/orclauses.h             |  2 +-
src/include/optimizer/paramassign.h           |  2 +-
src/include/optimizer/pathnode.h              |  2 +-
src/include/optimizer/paths.h                 |  2 +-
src/include/optimizer/placeholder.h           |  2 +-
src/include/optimizer/plancat.h               |  2 +-
src/include/optimizer/planmain.h              |  2 +-
src/include/optimizer/planner.h               |  2 +-
src/include/optimizer/prep.h                  |  2 +-
src/include/optimizer/restrictinfo.h          |  2 +-
src/include/optimizer/subselect.h             |  2 +-
src/include/optimizer/tlist.h                 |  2 +-
src/include/partitioning/partprune.h          |  9 +++++----
src/include/statistics/statistics.h           |  2 +-
src/include/utils/selfuncs.h                  |  2 +-
44 files changed, 60 insertions(+), 59 deletions(-)