feat: allow to split buildInputs in python derivations

Follow-up of https://github.com/nix-community/dream2nix/pull/676#discussion_r1326998054, now dependencies marked as buildInputs won't be included as propagatedBuildInputs for python derivations.

modules/dream2nix/pip/default.nix

@@ -11,6 +11,8 @@
 
   # filter out ignored dependencies
   targets = cfg.targets;
+  isRootDrv = drv: cfg.rootDependencies.${drv.name} or false;
+  isBuildInput = drv: cfg.buildDependencies.${drv.name} or false;
 
   writers = import ../../../pkgs/writers {
     inherit lib;
@@ -159,10 +161,21 @@ in {
   };
 
   mkDerivation = {
-    propagatedBuildInputs = let
-      rootDeps = lib.filterAttrs (_: x: x == true) cfg.rootDependencies;
+    buildInputs = let
+      rootDeps =
+        lib.filterAttrs
+        (name: value: isRootDrv value && isBuildInput value)
+        cfg.drvs;
     in
-      l.attrValues (l.mapAttrs (name: _: cfg.drvs.${name}.public.out) rootDeps);
+      l.map (drv: drv.public.out) (l.attrValues rootDeps);
+
+    propagatedBuildInputs = let
+      rootDeps =
+        lib.filterAttrs
+        (name: value: isRootDrv value && !isBuildInput value)
+        cfg.drvs;
+    in
+      l.map (drv: drv.public.out) (l.attrValues rootDeps);
   };
 
   public.pyEnv = let

modules/dream2nix/pip/interface.nix

@@ -59,6 +59,38 @@ in {
         list of requirements.txt files
       '';
     };
+    buildDependencies = l.mkOption {
+      type = t.attrsOf t.bool;
+      default = {
+        cython = true;
+        flit-core = true;
+        flit-scm = true;
+        hatch-fancy-pypi-readme = true;
+        hatch-nodejs-version = true;
+        hatch-vcs = true;
+        hatchling = true;
+        pbr = true;
+        pdm-pep517 = true;
+        poetry-core = true;
+        poetry-dynamic-versioning = true;
+        setuptools = true;
+        setuptools-odoo = true;
+        setuptools-scm = true;
+        versioneer = true;
+        wheel = true;
+      };
+      description = ''
+        python packages to be added only as buildInputs.
+        These should be somehow installable from `requirementsList` or
+        `requirementsFiles` too; listing them here doesn't do that automatically.
+      '';
+      example = lib.literalExpression ''
+        {
+          setuptools-scm = false; # To disable the default
+          easy_install = true; # To select easy_install as a buildInput
+        }
+      '';
+    };
 
     buildExtras = l.mkOption {
       type = t.listOf t.str;