Rename values of _ProjectRestoreType property

[dsplaisted]

NuGet.targets sets the value of the _ProjectRestoreType property based on how package references are expressed. However, the values of the property currently refer to what the project is targeting, which may not correspond to how package references are expressed. For example, it's possible to use PackageReference with .NET Framework projects, but the value of _PackageRestoreType would be NETCore.

So I suggest considering changing UAP to ProjectJson and NETCore to PackageReference.

[emgarten]

@dsplaisted what scenarios are you relying on this internal property for?

The work to expose this publicly is in progress and being tracked here: #3928 ProjectJson and PackageReference are the two possible values so far.

[dsplaisted]

See #4046. In the .NET SDK, we want the project restore type to be .NET Core, even if there aren't any PackageReferences in the project, so that we get the transitive package references from referenced projects. Right now we do this by injecting a target