Add _RestorePackagePruningDefault to nomination

[nkolev92]

Related to NuGet/NuGet.Client#6696.

Used for inferring pruning defaults.

Microsoft Reviewers: Open in CodeFlow

[drewnoakes]

I think this is the first example of a "private" (starting with underscore) variable being bound by the project system. My only concern here is that whoever defines this property decides it's safe to change on this basis, which breaks the contract here.

[lifengl]

@nkolev92 & @drewnoakes : private '_' properties are not saved in the project cache, so the value would be different when the solution loads from cache. saving private data would increase size of cache state and leads regressions.

[drewnoakes]

Thanks @lifengl, I didn't know they weren't cached, though it makes sense. I've added a unit test to catch this in future: #9745

@nkolev92 we should work out how to progress here so that we can remove the various workarounds we have put in place for this.

[nkolev92]

I can change it to avoid using an _. NuGet/Home#14511

I'll fix it soon.