_ prefixed top-level symbols now file-local

examples/packages/hidden_valid/app/a.bro

@@ -0,0 +1,22 @@
+_Thing :: struct {
+	value i32
+}
+
+_value :: 1
+
+_helper func() i32 {
+	thing _Thing = _Thing { value = _value }
+	return thing.value
+}
+
+_foreign c_func() i32 {
+	return 1
+}
+
+_C_Record :: c_struct {
+	value c_int
+}
+
+from_a func() i32 {
+	return _helper()
+}