builder: fix a bug with submodule import (#10318)

examples/submodule/main.v

@@ -0,0 +1,7 @@
+import mymodules { add_xy }
+import mymodules.submodule { sub_xy }
+
+fn main() {
+	println(add_xy(2, 3)) // expected: 5
+	println(sub_xy(10, 7)) // expected: 3
+}