How to reproduce:
1. Create a new 3D project
2. Go to Window -> Package Manager
3. Install the "Post Processing" package in the Package Manager window

Expected result: "Post Processing" package is installed
Actual result: "Built-in package [com.unity.postprocessing@3.0.3] cannot be found [NotFound]." error is thrown in the Console log

Reproducible with: 2021.1.0b4, 2021.1.0b8, 2021.2.0a6
Not reproducible with: 2018.4.32f1, 2021.1.0b3

Note:
- Adding the package through manifest.json also causes the same errors in the Console log