As per dotnet/source-build#886 we need to remove our source-build dependencies on NuGet packages.

Currently FsLex/FsYacc use FSharp.Core 4.6.2 when building src/buildtools/buildtools.proj and this needs to change.

Possible paths forward:

• 'Borrow' the FSharp.Core.dll that's alongside the CLI compiler used during source-build.
• Add a <ProjectReference> to src/fsharp/FSharp.Core/FSharp.Core.fsproj.