_ prefixed top-level symbols now file-local

examples/packages/hidden_valid/app/b.bro

@@ -0,0 +1,20 @@
+_Thing :: struct {
+	value i32
+}
+
+_value :: 2
+
+_helper func() i32 {
+	thing _Thing = _Thing { value = _value }
+	return thing.value
+}
+
+Box :: struct {
+	_value i32
+}
+
+from_b func(_input i32) i32 {
+	_local Box = Box { _value = _input }
+	record _C_Record = _C_Record { value = 0 }
+	return _helper() + _local._value + _foreign() + i32(record.value)
+}