Re-export module namespace as global

Nobody has claimed this yet.

We're attempting to migrate users within Google to new @types typings, which move most typings into modules, and having a ton of trouble. The basic problem is that a module (e.g. CodeMirror) used to declare a global complex interface:

```
declare function CodeMirror(...);
declare namespace CodeMirror { ... }
```

and now they are modules:

```
export = CodeMirror;
```

and other typings refer to them as modules (so we can't just undo that export statement) but we also have tons of older user code that *doesn't* refer to them as modules. Ideally we'd be able to make it work both ways while we migrate users incrementally.

I appreciate that you don't want the language to make this convenient, but I'm having difficulty finding any way to make it work temporarily while we migrate.

Some things I tried:

1) Aliasing the module into the global namespace
```
declare global {
import CodeMirror = require('codemirror');
}
```
(and other variants) fails with "Imports are not permitted in module augmentations.".

2) Making the module global first, then aliasing it into a module namespace.
If you wrap the file contents with a "declare global {", then I couldn't figure out a way to repackage that global as a module again. The `export = CodeMirror;` fails with "Cannot find name 'CodeMirror'." despite the global CodeMirror being declared in the same file.

Do you have any advice? Note that "export as namespace" doesn't help because our user code is already modules, not scripts.

Start with the two declaration patterns and the diagnostics quoted in the issue. Trace how `export =`, `declare global`, and `export as namespace` are handled, then determine whether a supported approach or language change can satisfy both module and global usage; done would require a clear, tested resolution.

Written by the indexing model from the issue text.