feat: treat default dependencies as main group

Poetry documentation on dependency groups [1] says:
  "The dependencies declared in tool.poetry.dependencies are part of an
implicit main group."

The current implementation does not account for that and in particular
does not allow to skip installation of default dependencies like in one
of
`poetry install --without main`
`poetry install --only-root`
`poetry install --only othergroup`.

This commit adds main as an implicit group and also adds it as a default
value where applicable.

[1]
https://python-poetry.org/docs/managing-dependencies#dependency-groups

Closes #1578

default.nix

@@ -31,7 +31,7 @@ let
     , pyProject
     , attrs
     , includeBuildSystem ? true
-    , groups ? [ ]
+    , groups ? [ "main" ]
     , checkGroups ? checkGroupsDefault
     , extras ? [ "*" ]  # * means all extras, otherwise include the dependencies for a given extra
     }:
@@ -82,12 +82,9 @@ let
     {
       buildInputs = mkInput "buildInputs" (if includeBuildSystem then buildSystemPkgs else [ ]);
       propagatedBuildInputs = mkInput "propagatedBuildInputs" (
-        getDeps allRawDeps ++ (
-          # >=poetry-1.2.0 dependency groups
-          if pyProject.tool.poetry.group or { } != { }
-          then lib.flatten (map (g: getDeps pyProject.tool.poetry.group.${g}.dependencies) groups)
-          else [ ]
-        )
+        let
+          availableGroups = {main.dependencies = allRawDeps;} // pyProject.tool.poetry.group or { };
+        in lib.flatten (map (g: getDeps availableGroups.${g}.dependencies) groups)
       );
       nativeBuildInputs = mkInput "nativeBuildInputs" [ ];
       checkInputs = mkInput "checkInputs" checkInputs';
@@ -145,7 +142,7 @@ lib.makeScope pkgs.newScope (self: {
       # Example: { my-app = ./src; }
     , editablePackageSources ? { }
     , pyProject ? readTOML pyproject
-    , groups ? [ ]
+    , groups ? [ "main" ]
     , checkGroups ? checkGroupsDefault
     , extras ? [ "*" ]
     }:
@@ -314,7 +311,7 @@ lib.makeScope pkgs.newScope (self: {
     , preferWheels ? false
     , editablePackageSources ? { }
     , extraPackages ? _ps: [ ]
-    , groups ? [ "dev" ]
+    , groups ? [ "main" "dev" ]
     , checkGroups ? checkGroupsDefault
     , extras ? [ "*" ]
     }:
@@ -332,13 +329,12 @@ lib.makeScope pkgs.newScope (self: {
         (pkg: editablePackageSources."${pkg}" == null)
         (builtins.attrNames editablePackageSources);
 
-      allEditablePackageSources = (getEditableDeps (pyProject.tool.poetry."dependencies" or { }))
-        // (getEditableDeps (pyProject.tool.poetry."dev-dependencies" or { }))
+      allEditablePackageSources = (getEditableDeps (pyProject.tool.poetry."dev-dependencies" or { }))
         // (
-        # Poetry>=1.2.0
-        if pyProject.tool.poetry.group or { } != { } then
-          builtins.foldl' (acc: g: acc // getEditableDeps pyProject.tool.poetry.group.${g}.dependencies) { } groups
-        else { }
+          let 
+            deps = pyProject.tool.poetry."dependencies" or { };
+            availableGroups = {main.dependencies = deps;} // pyProject.tool.poetry.group or { };
+          in builtins.foldl' (acc: g: acc // getEditableDeps availableGroups.${g}.dependencies) { } groups
       )
         // editablePackageSources;
 
@@ -380,7 +376,7 @@ lib.makeScope pkgs.newScope (self: {
     , python ? pkgs.python3
     , pwd ? projectDir
     , preferWheels ? false
-    , groups ? [ ]
+    , groups ? [ "main" ]
     , checkGroups ? checkGroupsDefault
     , extras ? [ "*" ]
     , ...