postgresql: replace pg_config with custom script

By replacing upstream's pg_config binary with a shell script, we:
- gain the ability to run pg_config easily when cross-compiling,
- can remove the fake pg_config in the default output,
- can remove the pg_config wrapper script dealing with special cases.

Some 20 years ago, pg_config *was* a shell script upstream, too. It was
changed to a binary, when it was made "relocatable", so it would return
paths depending on the location of the "postgres" binary. However, this
is exactly the thing that just hurts us in nixpkgs - we don't want those
paths to change, we want them to always point at the right outputs. By
writing the script ourselves, this becomes a lot less painful.

This approach means more lines of codes, but all of them are dead simple
and we have a lot less complexity overall.

Additionally, pg_config is now made a separate derivation, only exposed
as "postgresql.pg_config". This has the nice side-effect, that all users
of postgresql and libpq in nixpkgs must be very *explicit* about their
dependency on pg_config. This gives a lot more visibility into the state
of affairs regarding pkg-config support for libpq, which ultimately is
the much better solution.

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