An executable is more robust to use than shell aliases.
This is also a preparation for commit 'add module test' because the
NixOS testing framework makes interactive aliases hard to use: It
unsets 'PS1' which is used by programs/bash/bash.nix to detect
interactive shells.
Not polluting the main pkgs namespace with internal pkgs makes it
easier to integrate the nix-bitcoin modules into a larger config.
Also, by overriding the nix-bitcoin namespace, users can now easily set the
packages used by services that offer no explicit `package` option, like `clightning`.
