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.huinker@gmail.com>
Co-authored-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/aQ2k8--a0FfwSwX9@paquier.xyz