Extract the netstandard facades for FX projects out of the SDK into an implicitly loaded NuGet package

[MichaelSimons]

These facades are located in the Microsoft/Microsoft.NET.Build.Extensions folder of a .NET SDK installation.

net461/: 97 files (2.9 MB)
net462/: 1 file (36 KB)
net47/: 3 files (108 KB)
net471/: 14 files (736 KB)
Total: 115 files (3.73 MB)

These facades are included in all platforms (e.g. linux/macos). The inclusion of these in the SDK by default is less than ideal for numerous usage scenarios and over time their usage will decline as more customers migrate off of FX.

With the work being done in #41128, the size of these is going to be more noticeable.

[MichaelSimons]

cc @baronfel, @rainersigwald, @dsplaisted

[baronfel]

It would be ideal if these could be extracted out to a package similar to the .NET Framework Targeting Pack and be conditionally added as PackageReferences for .NET Framework applications that need these .NET Standard facade assemblies. That would be a bit larger of a cost in terms of product-construction, but it would let us pretty easily get out of this business altogether.

[rainersigwald]

I think one complexity is identifying the applications "that need these .NET Standard facade assemblies". It's difficult to know at pre-restore time whether some selected asset will be netstandard and thus need the facades.