feat(source-build): support for flat conditional exports

[chenjiahan]

Summary

Support for conditional exports without subpaths.

// package.json
{
  "exports": {
    "source": "./index-module.ts",
    "import": "./index-module.js",
    "require": "./index-require.cjs"
  },
  "type": "module"
}

Related Links

#2687

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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