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

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 https://github.com/dotnet/sdk/issues/41128, the size of these is going to be more noticeable.