workspace: Add support for using workspaceRoot from attrset with outPath

As returned from things like `builtins.fetchGit` & other builtin fetchers.

Closes #173

Author: adisbladis

lib/test_workspace.nix

@@ -153,4 +153,36 @@ in
       };
     };
 
+  # Test workspaceRoot passed as an attrset
+  # This is analogous to using builtins.fetchGit & such which return an attrset with an outPath member.
+  loadWorkspace.fetchedWorkspace =
+    let
+      mkTestSet =
+        workspaceRoot:
+        let
+          ws = workspace.loadWorkspace { inherit workspaceRoot; };
+
+          overlay = ws.mkPyprojectOverlay { sourcePreference = "wheel"; };
+
+          pythonSet =
+            (pkgs.callPackage pyproject-nix.build.packages {
+              python = pkgs.python312;
+            }).overrideScope
+              overlay;
+        in
+        pythonSet;
+
+      wsRoot = {
+        outPath = "${./fixtures/workspace-flat}";
+      };
+      testSet = mkTestSet wsRoot;
+    in
+    {
+      # Test that the stringly src lookup is correct relative to the workspace root
+      testOutpathSrc = {
+        expr = testSet."pkg-a".src == "${wsRoot}/packages/pkg-a";
+        expected = true;
+      };
+    };
+
 }

lib/workspace.nix

@@ -95,7 +95,11 @@ fix (self: {
       # - A function taking the generated config as an argument, and returning the augmented config
       config ? { },
     }:
-    assert (isPath workspaceRoot || (isString workspaceRoot || hasContext workspaceRoot));
+    assert (
+      isPath workspaceRoot
+      || (isString workspaceRoot && hasContext workspaceRoot)
+      || (isAttrs workspaceRoot && workspaceRoot ? outPath)
+    );
     assert isAttrs config || isFunction config;
     let
       pyproject = importTOML (workspaceRoot + "/pyproject.toml");