How to reproduce:
1. Create a new project
2. Go to Window -> Package Manager
3. Click on Advanced and tick 'Show preview packages'
4. Install 'Build Report Inspector' package

Expected results: Build Report Inspector installs successfully
Actual results: Errors regarding missing .meta files occur in console

Reproducible with: 2018.4.12f1, 2019.3.0b10, 2020.1.0a12 (Package version preview - 0.1.0)
Not reproducible with 2019.2.0a1, 2019.3.0b9
Could not test with 2019.2.11f1, 2020.1.0a1 due to other errors when installing the package

Errors:
"A meta data file (.meta) exists but its asset 'Packages/com.unity.build-report-inspector/package-lock.json' can't be found. When moving or deleting files outside of Unity, please ensure that the corresponding .meta file is moved or deleted along with it."
"Couldn't delete Packages/com.unity.build-report-inspector/package-lock.json.meta because it's in an immutable folder."
"Asset Packages/com.unity.build-report-inspector/Packages has no meta file, but it's in an immutable folder. The asset will be ignored."
"Asset Packages/com.unity.build-report-inspector/Packages/manifest.json has no meta file, but it's in an immutable folder. The asset will be ignored."

Notes:
- The errors re-occur whenever the Editor is updated (for i.e. when creating a new GameObject)
- In versions where the issue is not present, when installing the package, only the first warning appears