Change cssmodule classname hash to use repo relative paths (#6876)

* Change cssmodule classname hash to use repo relative paths instead of system absolute paths

packages/react-dev-utils/getCSSModuleLocalIdent.js

@@ -8,6 +8,7 @@
 'use strict';
 
 const loaderUtils = require('loader-utils');
+const path = require('path');
 
 module.exports = function getLocalIdent(
   context,
@@ -23,7 +24,7 @@ module.exports = function getLocalIdent(
     : '[name]';
   // Create a hash based on a the file location and class name. Will be unique across a project, and close to globally unique.
   const hash = loaderUtils.getHashDigest(
-    context.resourcePath + localName,
+    path.posix.relative(context.rootContext, context.resourcePath) + localName,
     'md5',
     'base64',
     5