[next] Re-enable shared lambdas by default (#4757)

This re-enables the shared lambdas optimization by default and also adds additional tests to ensure that when a `now.json` or `vercel.json` contains either `routes` or `functions` configs the shared lambda optimization is disabled. 

Additional test deployments done:

- minimal `now.json` with `builds` config [deploy](https://shared-lambdas-tests-d646fsqju.vercel.app)
- `now.json` with `functions` config [deploy](https://shared-lambdas-tests-ahnuosp4s.vercel.app)
- `now.json` with `routes` config [deploy](https://shared-lambdas-tests-gulam3jda.vercel.app)
- minimal `vercel.json` with `builds` config [deploy](https://shared-lambdas-tests-7ic7wzirs.vercel.app)
- `vercel.json` with `functions` config [deploy](https://shared-lambdas-tests-7ic7wzirs.vercel.app)
- `vercel.json` with `routes` config [deploy](https://shared-lambdas-tests-rja2391tq.vercel.app)

test/lib/deployment/now-deploy.js

@@ -5,9 +5,11 @@ const _fetch = require('node-fetch');
 const fetch = require('./fetch-retry.js');
 const sleep = ms => new Promise(resolve => setTimeout(resolve, ms));
 
-async function nowDeploy(bodies, randomness) {
+async function nowDeploy(bodies, randomness, uploadNowJson) {
   const files = Object.keys(bodies)
-    .filter(n => n !== 'vercel.json' && n !== 'now.json')
+    .filter(n =>
+      uploadNowJson ? true : n !== 'vercel.json' && n !== 'now.json'
+    )
     .map(n => ({
       sha: digestOfFile(bodies[n]),
       size: bodies[n].length,