Support goog.require calls by overloading the function

[rkirov]

To typecheck user code, written in closure module style:

var {trim} = goog.require('goog.string');

we can do so by emitting:

declare namespace ಠ_ಠ.cl2dts_internal.goog {
  function require(name: string): any;
  function require(name: 'goog.string'): typeof ಠ_ಠ.cl2dts_internal.goog.string;
}