Make overrides export the overlay function

This is preparation work for providing a function that allows
customising the overrides.

default.nix

@@ -11,7 +11,7 @@ let
   version = "1.0.0";
 
   /* The default list of poetry2nix override overlays */
-  defaultPoetryOverrides = [ (import ./overrides.nix { inherit pkgs; }) ];
+  defaultPoetryOverrides = (import ./overrides.nix { inherit pkgs lib; });
 
   mkEvalPep508 = import ./pep508.nix {
     inherit lib;
@@ -35,7 +35,7 @@ let
   mkPoetryPython =
     { poetrylock
     , poetryPkg
-    , overrides ? defaultPoetryOverrides
+    , overrides ? [ defaultPoetryOverrides ]
     , meta ? {}
     , python ? pkgs.python3
     , pwd ? null
@@ -115,7 +115,7 @@ let
   */
   mkPoetryEnv =
     { poetrylock
-    , overrides ? defaultPoetryOverrides
+    , overrides ? [ defaultPoetryOverrides ]
     , meta ? {}
     , pwd ? null
     , python ? pkgs.python3
@@ -135,7 +135,7 @@ let
     { src
     , pyproject
     , poetrylock
-    , overrides ? defaultPoetryOverrides
+    , overrides ? [ defaultPoetryOverrides ]
     , meta ? {}
     , python ? pkgs.python3
     , pwd ? null

tests/git-deps-pinned/default.nix

@@ -6,7 +6,8 @@ poetry2nix.mkPoetryApplication {
   poetrylock = ./poetry.lock;
   src = lib.cleanSource ./.;
 
-  overrides = poetry2nix.defaultPoetryOverrides ++ [
+  overrides = [
+    poetry2nix.defaultPoetryOverrides
     (import ./poetry-git-overlay.nix { inherit pkgs; })
   ];
 

tests/override-support/default.nix

@@ -6,7 +6,8 @@ let
     src = ./.;
     poetrylock = ./poetry.lock;
     pyproject = ./pyproject.toml;
-    overrides = poetry2nix.defaultPoetryOverrides ++ [
+    overrides = [
+      poetry2nix.defaultPoetryOverrides
       (
         self: super: {
           alembic = super.alembic.overrideAttrs (