pgsql: Move code specific to pg_dependencies to new file

Move code specific to pg_dependencies to new file

This new file is named pg_dependencies.c and includes all the code
directly related to the data type pg_dependencies, extracted from the
extended statistics code.

Some patches are under discussion to change its input and output
functions, and this separation makes the follow-up changes cleaner by
separating the logic related to the data type and the functional
dependencies statistics core logic in dependencies.c.

Author: Corey Huinker <corey(dot)huinker(at)gmail(dot)com>
Co-authored-by: Michael Paquier <michael(at)paquier(dot)xyz>
Discussion: https://postgr.es/m/aQ2k8--a0FfwSwX9@paquier.xyz

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/2ddc8d9e9baae28ba63a9f1c85f0a36deb9112aa

Modified Files
--------------
src/backend/statistics/dependencies.c | 91 ----------------------------
src/backend/utils/adt/Makefile | 1 +
src/backend/utils/adt/meson.build | 1 +
src/backend/utils/adt/pg_dependencies.c | 104 ++++++++++++++++++++++++++++++++
4 files changed, 106 insertions(+), 91 deletions(-)