Each secret file to be deployed is now backed by one local file.
This simplifies 'setup-secrets' and the secret definitions.
Also, with the old format it was not possible to add new secrets
to secrets.nix in a simple way.
Old secrets are automatically converted to the new format when running
nix-shell.
Using the new option 'nix-bitcoin.secrets', secrets are now directly
defined by the services that use them.
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`.
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.
