Use implicit "start" script when `server.js` exists

Moves the verification step to the `create()` function,
instead of in `readMetaData()`.

Closes https://github.com/zeit/now-cli/issues/181.

lib/index.js

@@ -23,6 +23,9 @@ const Agent = require('./agent');
 const readMetaData = require('./read-metadata');
 const toHost = require('./to-host');
 
+// Helpers
+const { error } = require('./error');
+
 // How many concurrent HTTP/2 stream uploads
 const MAX_CONCURRENT = 10;
 
@@ -77,6 +80,18 @@ module.exports = class Now extends EventEmitter {
     const opts = { debug: this._debug, hasNowJson };
     if (deploymentType === 'npm') {
       files = await getNpmFiles(path, pkg, nowConfig, opts);
+
+      // A `start` or `now-start` npm script, or a `server.js` file
+      // in the root directory of the deployment are required
+      if (!hasNpmStart(pkg) && !hasFile(path, files, 'server.js')) {
+        error(
+          'Missing `start` (or `now-start`) script in `package.json`. ' +
+            'See: https://docs.npmjs.com/cli/start.'
+        );
+
+        // eslint-disable-next-line unicorn/no-process-exit
+        process.exit(1);
+      }
     } else {
       files = await getDockerFiles(path, nowConfig, opts);
     }
@@ -989,3 +1004,12 @@ async function responseError(res) {
 
   return err;
 }
+
+function hasNpmStart(pkg) {
+  return pkg.scripts && (pkg.scripts.start || pkg.scripts['now-start']);
+}
+
+function hasFile(base, files, name) {
+  const relative = files.map(file => toRelative(file, base));
+  return relative.indexOf(name) !== -1;
+}

lib/read-metadata.js

@@ -84,19 +84,6 @@ async function readMetaData(
       }
     }
 
-    if (
-      strict &&
-      (!pkg.scripts || (!pkg.scripts.start && !pkg.scripts['now-start']))
-    ) {
-      error(
-        'Missing `start` (or `now-start`) script in `package.json`. ' +
-          'See: https://docs.npmjs.com/cli/start.'
-      );
-
-      // eslint-disable-next-line unicorn/no-process-exit
-      process.exit(1);
-    }
-
     if (!deploymentName) {
       if (typeof pkg.name === 'string' && pkg.name !== '') {
         name = pkg.name;