How to reproduce:
1. Download the attached 'PackageProject.zip' project
2. Open a new project
3. Open Package Manager
4. In Package Manager select '+' icon and 'Add package from disk...'
5. In File explorer select 'PackageProject' > 'Assets' > 'package'
6. In Project window select 'Packages' > 'Sarepta Framework' (might need to re-open the project for the package to be visible)
7. Inspect 'PackageAssembly.asmdef' file

Expected result: 'PackageAssembly.asmdef' file is not corrupted
Actual result: 'PackageAssembly.asmdef' file is corrupted

Reproducible with: 2018.4.5f1, 2019.1.13f1, 2019.2.0f1, 2019.3.0a11
Could not test with: 2017.4.30f1 and earlier (no package manager)

Additional notes:
1) .amsdef files from pre-built packages are not corrupted
2) due to the assembly files being corrupted, you are not able to reference scripts from the custom package