How to reproduce:
1. Download the attached "1263378 repro.zip" project
2. Open "manifest.json" and "packages-lock.json" files from the Packages folder and observe that they are empty
3. Select "manifest.json" and "packages-lock.json" files in the Packages folder
4. In the right-click menu click Properties and make both files Read-only
5. Open the project with a newer version of Unity than the one provided

Expected results: Package Manager resolver doesn't ask for writable access to "manifest.json" and "packages-lock.json" files
Actual results: Package Manager resolver asks for writable access to "manifest.json" and "packages-lock.json" files

Reproducible with: 2018.4.25f1, 2019.4.5f1, 2020.1.1f1, 2020.2.0a19

Notes:
-This issue can also be reproduced with default project packages
-Package Manager resolver usually doesn't modify these files after giving it writable access
-Usually, the Package Manager resolver actually modifies these files only when installing the "com.unity.ide.visualstudio" package in newer versions of Unity