pgsql: Cosmetic improvements for code related to partitionwise join.

Cosmetic improvements for code related to partitionwise join.

Move have_partkey_equi_join and match_expr_to_partition_keys to
relnode.c, since they're used only there.  Refactor
build_joinrel_partition_info to split out the code that fills the
joinrel's partition key lists; this doesn't have any non-cosmetic
impact, but it seems like a useful separation of concerns.
Improve assorted nearby comments.

Amit Langote, with a little further editorialization by me

Discussion: https://postgr.es/m/CA+HiwqG2WVUGmLJqtR0tPFhniO=H=9qQ+Z3L_ZC+Y3-EVQHFGg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0568e7a2a4f133a7c16776bcae92c53fcf247b73

Modified Files
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src/backend/optimizer/path/joinrels.c | 168 --------------------
src/backend/optimizer/util/plancat.c  |  14 +-
src/backend/optimizer/util/relnode.c  | 279 +++++++++++++++++++++++++++++-----
src/include/nodes/pathnodes.h         |  53 ++++---
src/include/optimizer/paths.h         |   3 -
5 files changed, 283 insertions(+), 234 deletions(-)