nix: package bases as inputs

This is to enable call customization, typically to change the
base pybatsim packages to use without needing to change default.nix
but only by giving the right arguments to nix-build.

default.nix

 { kapack ? import
     (fetchTarball "https://github.com/oar-team/nur-kapack/archive/master.tar.gz")
   {}
+, pybatsim-core-base ? kapack.pybatsim-core
+, pybatsim-functional-base ? kapack.pybatsim-functional
 }:
 
 let
   self = rec {
-    pybatsim-core = kapack.pybatsim-core.overrideAttrs (attrs: rec {
+    pybatsim-core = pybatsim-core-base.overrideAttrs (attrs: rec {
       name = "${attrs.name}-local";
       src = kapack.pkgs.lib.sourceByRegex ./pybatsim-core [
         "^pyproject\.toml$"
@@ -26,7 +28,7 @@ let
         "^src/pybatsim/schedulers/unMaintained/.\+\.py$"
       ];
     });
-    pybatsim-functional = kapack.pybatsim-functional.overrideAttrs (attrs: rec {
+    pybatsim-functional = pybatsim-functional-base.overrideAttrs (attrs: rec {
       name = "${attrs.name}-local";
       src = kapack.pkgs.lib.sourceByRegex ./pybatsim-functional [
         "^pyproject\.toml$"