How to reproduce:
1. Create a new project in your prefered workspace
2. Connect it to Perforce
3. Open the Package Manager and add a package from disk (For example, render pipeline core from: https://github.com/Unity-Technologies/ScriptableRenderPipeline)
4. In the Project Window, open the added package's folder and select any of its scripts

Expected result: no errors are thrown
Actual result: errors about the script being outside of the client's root are constantly thrown and the Editor performance is hampered

Reproduced in: 2019.3.0a11, 2019.3.0a6
Not reproducible in: 2019.3.0a5, 2019.3.0a1, 2019.2.0f1, 2019.1.13f1, 2018.4.5f1