Modularize `shell.nix`-es

[toraritte]

shell.nix-es can be curried (i.e., being called one after another) without any issues (that I'm aware of) so instead creating highly specific Nix expressions, create many smaller ones doing one task only that can be composed easier.

For example, there is the (Elixir-Phoenix-Postgres shell.nix)[https://github.com/toraritte/shell.nixes/blob/main/elixir-phoenix-postgres/shell.nix], my go-to Nix expression when trying to set PostgreSQL up quickly in other contexts. Spent a lot of time figuring out the PostgreSQL-specific parts - and it is mostly that anyway. Now I need to fire up a legacy Django project, but only need PostgreSQL, not the rest.

[toraritte]

This issue may have been solved; see #1.

[toraritte]

Nope, almost total failure. It is hard (for me) to make relative paths (i.e., the Nix data type) to properly resolve in different context. Anyway, wasted too much time on it, learned a lot, the last commit on main works (however broken it is), but will revisit this later on.

Advice to future self: Don't obsess on having the scripts working locally (i.e., from the cloned repo); that is only important for testing, and the whole point of this exercise that the shell.nix-es could be called remotely.