Missing .meta file for OptionalExtras/Async

[MrCheez]

Describe the bug
The .meta file for OptionalExtras/Async is missing. This is a breaking issue for everyone who imports Extenject via UPM.

To Reproduce

1. Create or checkout a new project
2. Add https://github.com/svermeulen/Extenject.git?path=/UnityProject/Assets/Plugins/Zenject/ to your project manifest
3. Open your project
4. Witness Asset Packages/com.svermeulen.extenject/OptionalExtras/Async has no meta file, but it's in an immutable folder. The asset will be ignored.

Screenshots

Extenject and Unity info (please complete the following information):

• Zenject version: every version since commit 6851a53
• Unity version: 2020.2

[T-Kuhn]

Please fix this.