Make private runtime-only dependencies on .NET Standard 2.0.

[teo-tsirpanis]

Context

Since #6148 we produce only reference assemblies for .NET Standard 2.0, which means that several of the our NuGet packages' dependencies when targeting .NET Standard 2.0 are unused.

Changes Made

The project files for Microsoft.Build.Framework, Utilities.Core and Tasks were updated to apply PrivateAssets="all" to all package references that are not exposed in the package's public API, when targeting .NET Standard 2.0.

Testing

I ran dotnet pack on these projects and validated manually that on .NET Standard 2.0, Microsoft.Build.Framework has zero dependencies, Utilities.Core depends only on Framework, and Tasks depends only on the previous two.

Because Roslyn keeps some internal APIs in reference assemblies, these reference assemblies still reference some assemblies whose respective package is not depended upon. I manually validated with ILSpy that the types in these assemblies are used only by internal APIs.

Notes

This is going to be a (minor) source-breaking change if a .NET Standard 2.0 project uses APIs from on one of the removed packages and transitively depended on it. They will have to directly depend on them, and it's not the first time we do a change like this (#9055).

I don't think that this is a binary-breaking change because the .NET Standard 2.0 binaries are not being used at runtime.

[rainersigwald]

Nice, I've been planning to do something like this (and likely even going further). Will look shortly.

[teo-tsirpanis]

and likely even going further

Let me know how further this can go, happy to do it now or in a subsequent PR.