Lint for unexported types on exported types

[dsherret]

Example failures:

interface SomeInterface {
}

export function myFunc(opts: SomeInterface ) { // fail
}

export class Test implements SomeInterface { // fail
  myMethod(): SomeInterface { // fail
  }
}

/// etc...

This would be a good first start... for a second pass we could maybe consider implementing cross file lints. For example:

// mod.ts
export { myFunc } from "./other.ts"; // fail

// other.ts

export interface MyArg {
 // ...
}

export function myFunc(arg: MyArg) {
  // ...
}

[magurotuna]

This sounds like a reasonable lint rule. I'll implement it.