Disallow global namespace pollution: External ambient module declaration should not override other external module typings

[falsandtru]

Ambient module declaration scope should be only own module(node package).

// node_modules/foo/types.d.ts
declare module 'foo' {
  export var fizz;
}
declare module 'bar' {
  export var fizz;
}

// node_modules/foo/index.d.ts
import './types.d';
export * from 'foo'; // should need re-exporting of ambient module for exporting as an external module.
                     // or should export only module named by own module(node package) name, but it is weak to typo.

// node_modules/bar/index.d.ts
export var bazz;

// index.ts
import {fizz} from 'foo';
import {bazz} from 'bar'; // should not be type error, this is incorrect error.

bar self hosted typings overridden by ambient module declarations in foo. This is global namespace pollution. Global namespace management should be systematic.

[mhegazy]

While i do agree that global namespace pollution is bad, it does exist and ppl build large scale applications and libraries on top of that. i think there is enough user feedback to allow/regulate more of this side effects rather than to limit it. see #5269 and #5867 to name a few..

[falsandtru]

All right, thanks.