I just installed Unity 2019.2 alpha 6 and I receive this error message now:
Library\PackageCache\com.unity.visualeffectgraph@5.3.1-preview\Editor\PackageInfo.cs(20,41): error CS0234: The type or namespace name ‘Packages’ does not exist in the namespace ‘UnityEditor.PackageManager’ (are you missing an assembly reference?)
So there seems to be some problem with PackageInfo. Shader Graph and HDRP are both version 5.3.1
Staging makes your package manager to seek the packages from staging registry. It’s not enabled by default as these packages haven’t passed QA checks yet. It’s meant for testing purposes only. This is also why you see a lot of packages on staging that never make it to the regular registry.