Extracts some useful parts of NixOS#38698,
in particular, it's vision that postgresql plugins should be namespaced.

Original approach had several problems:
- not gonna happen in forseeable future
- did lots of deprecations
- was all-in-one solution, which is hard to sell to nixpkgs
- even that we have postgresqlPackages now, we can't do arbitrary overrides
  to postgresql and plugins. Several required functions were not exported

Here I've fixed all of those problems:
- deprecates nothing (though plugins were moved now into `aliases.nix`)
- this doesn't touch NixOS at all, and doesn't break anything
- hashes for plugins and PGs are not changed (I hope)
- no extra fixes to pg itself
- default PG is not changed
- plugins and PGs are extensible

Last one is the most interesting thing, because it introduces novel way
to manage `XXX withPackages` problem. It is novel, but has got lots of
inspiration from existing approaches:
- python, so we have now `postgresql.pkgs.*`, `postgresql_11.pkgs.*`
  which all contain plugins compiled with correct PG.
- python, so we have now `postgresql.withPackages` as well
- in contrast to python, there are no `postgresql11Packages`, only
  `postgresql_11.pkgs`
- NixOS#44196, so plugins are referenced starting at self-fixpoint.
  This allows override/add plugins with mere `//` in overlay. This works for
  both `postgresqlPackages` (overrides are applied to all postgresql_xx.pkgs)
  and `postgresql_xx.pkgs` (overrides are specific to this postgresql) sets
- I've made it compatible with proposed mergeable overlays (NixOS#54266)
  however this PR doesn't depend on it
- last, but not least, `postgresql/default.nix` is now an overlay! This
  replaces previous `callPackages` approach with a modern, extensible concept.

Previous approach turned out to be awful. It was impossible to perform
deep override.

This time I didn't invent bycicles and used overlays for subpackages.

Big change is that now overriding `postgresqlPackages` doesn't override
all other package sets. But `postgresqlPackages` are now also available
as an overlay! So you can get that, reorganize whatever you want and then
attach to some postgresql.