Create type definition for '*/package.json' (#4014)

* Create type definition for '*/package.json'

* Update tsconfig.json

src/config/load.ts

@@ -7,7 +7,7 @@ import { URL } from 'url';
 import * as yaml from 'js-yaml';
 import { Source, Mixin } from './types';
 import isUrl = require('is-url');
-const pkg = require('../../package.json');
+import * as pkg from '../../package.json';
 
 /**
  * Path of configuration directory