fix(gomod): Fix gomod digest updates (#12655)

lib/datasource/go/digest.spec.ts

 import * as httpMock from '../../../test/http-mock';
 import { mocked } from '../../../test/util';
 import * as _hostRules from '../../util/host-rules';
-import { getDigest } from './digest';
+import { getDigest } from '.';
 
 jest.mock('../../util/host-rules');
 

lib/datasource/go/index.ts

@@ -4,6 +4,8 @@ import * as goproxy from './releases-goproxy';
 
 export { id } from './common';
 
+export { getDigest } from './digest';
+
 export const customRegistrySupport = false;
 
 export function getReleases(