pgsql: Reorganize planner code moved in b60c39759908

Reorganize planner code moved in b60c39759908

It seems modules are better defined like this instead of the original
split.

Per complaints from David Rowley as well as Amit Langote's self review.
Discussion: https://postgr.es/m/CAKJS1f988rsyhwvLgfT-y1UCYUfXDOv67ENQk=v24OxhsZOzZw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d723f56872a9fd1b898b7ee29ca5d26a9e538058

Modified Files
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src/backend/optimizer/path/joinrels.c   |  45 ++++++++
src/backend/optimizer/util/appendinfo.c | 185 ++++++++------------------------
src/backend/optimizer/util/inherit.c    |  55 ++++++++++
src/include/optimizer/appendinfo.h      |  14 +--
4 files changed, 148 insertions(+), 151 deletions(-)