How to reproduce:
1. Create and open a new Unity project
2. Download the attached "External.tgz" package
3. In the created project's Project folder create a new folder and put the downloaded package there
4. In the Editor, open the Package Manager (Window -> Package Manager)
5. Press the Plus button at the top-left of the Package Manager and select "Add package from tarball"
6. In the window that opens select the "External.tgz" package and press Open
7. Open the manifest.json (Project folder -> Packages -> manifest.json)

Expected results: The package in the manifest.json is listed as: file:../ExternalPackages/External.tgz
Actual results: The package in the manifest.json is listed as: file:DRIVE:/FOLDER/PROJECT/ExternalPackages/External.tgz

Reproducible with: 2020.3.1f1, 2020.3.4f1, 2021.1.3f1, 2021.2.0a13
Not reproducible with: 2019.4.23f1, 2020.3.0f1
Could not test with: 2018.4.34f1 (Adding a package via tarball is not possible)