filterPath: Ensure /nix/store/<hash>-<hash>-source

Issue NixOS#10627.

src/libexpr/primops.cc

@@ -2376,7 +2376,18 @@ static void prim_filterSource(EvalState & state, const PosIdx pos, Value * * arg
         "while evaluating the second argument (the path to filter) passed to 'builtins.filterSource'");
     state.forceFunction(*args[0], pos, "while evaluating the first argument passed to builtins.filterSource");
 
-    addPath(state, pos, path.baseName(), path, args[0], FileIngestionMethod::Recursive, std::nullopt, v, context);
+    addPath(
+        state,
+        pos,
+        path.path.isRoot()
+        ? state.store->computeStorePath("source", *path.accessor, path.path).first.to_string()
+        : path.baseName(),
+        path,
+        args[0],
+        FileIngestionMethod::Recursive,
+        std::nullopt,
+        v,
+        context);
 }
 
 static RegisterPrimOp primop_filterSource({