ci: expose fetchPipMetadata package and build it

v1/nix/modules/flake-parts/packages.fetchPipMetadata.nix

@@ -0,0 +1,18 @@
+# evaluate packages from `/**/modules/drvs` and export them via `flake.packages`
+{
+  self,
+  inputs,
+  ...
+}: {
+  perSystem = {
+    system,
+    config,
+    lib,
+    pkgs,
+    ...
+  }: {
+    packages.fetchPipMetadata = pkgs.callPackage ../../pkgs/fetchPipMetadata/package.nix {
+      python = pkgs.python3;
+    };
+  };
+}

v1/nix/pkgs/fetchPipMetadata/fetchPipMetadata.nix

@@ -49,7 +49,7 @@
   path = [nix git] ++ nativeBuildInputs;
 
   package = import ./package.nix {
-    inherit lib python;
+    inherit git lib python;
   };
 
   args = writeText "pip-args" (builtins.toJSON {

v1/nix/pkgs/fetchPipMetadata/package.nix

@@ -4,12 +4,14 @@
   # Pip needs to be executed from that specific python version.
   # Pip accepts '--python-version', but this works only for wheel packages.
   python,
+  git,
 }: let
   package = python.pkgs.buildPythonPackage {
     name = "fetch_pip_metadata";
     format = "flit";
     src = ./src;
     nativeBuildInputs = [
+      git
       python.pkgs.pytestCheckHook
     ];
     propagatedBuildInputs = with python.pkgs; [

v1/nix/pkgs/fetchPipMetadata/src/shell.nix

@@ -7,6 +7,7 @@
   python = pkgs.python3;
   package = import ../package.nix {
     inherit lib python;
+    inherit (pkgs) git;
   };
   pythonWithDeps = python.withPackages (
     ps: