If compiling external modules, with --declaration, each module gets its own declaration file. this is not useful for library authors, who probably want to hand off a single .d.ts file for all their modules. the only way to do this now, is to either hand edit the file, or use a tool to concatenate the output.

This becomes even more desirable with ES6 module export/import where a module can reexport exports of another module.

See discussion in #2516 (comment) for more details.