Avoid adding flake resource paths to the store (via string
interpolation).
This reduces performance and can lead to modules getting imported
twice, once through a local path and once through a store path.
This might not be needed in a future Nix release, in which case we can
revert this.
The strategy of invoking node2nix inside a derivation (installPhase in
this case) does not work, as under NixOS installations there is no
network traffic allowed during a derivation build. Hence, we move
node2nix outside and rewrite the packaging into the modules.
Also switch to callPackage instead of plain imports. This could
probably be done on all other imported packages inside of
nix-bitcoin-pkgs.nix.
