[Question] Sub-exports depending on main export bundle the entire main-export. #1027

I have a project where I have a main-export and multiple other exports. The other exports are extensions that depend on the main-export, but I don't want them to be in the main-export to keep it as small as possible.

Users should be able to do:

import {Thing} from 'package';

// optional addition
import {OtherThing} from 'package/a';

(or similarly using multiple script-tags with the umd-builds)

As for the source-files, it's roughly like this:

// index.ts
export * from './main';

// main.ts
export class Thing {}

// a.ts
import {Thing} from './main';

export class OtherThing extends Thing {}

And the package.json contains these sub-exports:

"exports": {
    ".": {
      "import": "./dist/index.modern.js",
      "require": "./dist/index.js",
      "umd": "./dist/index.umd.js",
      "browser": "./dist/index.module.js"
    },
    "./a": {
      "import": "./dist/a.modern.js",
      "require": "./dist/a.js",
      "umd": "./dist/a.umd.js",
      "browser": "./dist/a.module.js"
    },
  }

With this, the output-files for './a' (all formats) have the entire definition of './main.ts' inlined.

So my question is, is there a way to configure microbundle to treat those other files as externals?