Add preflight check to guard against wrong versions of webpack/eslint/jest higher up the tree (#3771)

* Run real scripts in local development

* Add preflight check warning

* I know what I am doing

* Move preflight check into individual scripts

This ensures we don't try to filter NODE_PATH twice, accidentally removing the now-absolute path.

* Slightly tweak the wording

* Fix lint

packages/react-scripts/scripts/start.js

@@ -21,6 +21,13 @@ process.on('unhandledRejection', err => {
 
 // Ensure environment variables are read.
 require('../config/env');
+// @remove-on-eject-begin
+// Do the preflight check (only happens before eject).
+const verifyPackageTree = require('./utils/verifyPackageTree');
+if (process.env.SKIP_PREFLIGHT_CHECK !== 'true') {
+  verifyPackageTree();
+}
+// @remove-on-eject-end
 
 const fs = require('fs');
 const chalk = require('chalk');