How to reproduce:
1. Download the attached "1263361 repro" project
2. Open "manifes.json" from the Packages folder
3. Change "com.unity.timeline" version from 1.2.14 to 1.2.15 and save the file
4. Select "manifest.json" and "packages-lock.json" files in the Packages folder
5. In the right-click menu click Properties and make both files Read-only
6. Open the project

Expected result: UPM asks if the user would like to make the required files writable in order to resolve the conflict
Actual result: resolving the packages fails without an option to make the files writable

Reproducible with: 2019.4.5f1, 2020.1.0b16, 2020.2.0a18
Could not test with: 2018.4.25f1 (the packages-lock.json doesn't exist)

Notes:
-When upgrading the project to a newer Unity version UPM asks whether it should make the files writable in order to resolve packages as expected