Use index.d.ts only (not foo/foo.d.ts) (#12834)

* Use index.d.ts only (not foo/foo.d.ts)

* Convert more packages

* Remove unnecessary references

webpack-dev-server/tsconfig.json

@@ -13,7 +13,7 @@
         "forceConsistentCasingInFileNames": true
     },
     "files": [
-        "webpack-dev-server.d.ts",
+        "index.d.ts",
         "webpack-dev-server-tests.ts"
     ]
 }