This reverts commit 63872019.

This package now only supports the TFM. This is because the purpose of the package
to layout a .NET Core executable, which requires the netcoreapp references, including CoreCLR itself.

There are also numerous references that are now provided by inheritance from the compiler itself, so
those references have been removed from the Microsoft.Net.Compilers.netcore nuspec.

Fixes #12674

This change drops support for .NET 4.5 and retargets to desktop
4.6 + CoreCLR. It also fixes up various analyzer warnings that
were created due to the move.

Fixes #12926

These facades only need to be present next to the build task, which is
in the MSBuild/Compiler authoring.

Contains the Roslyn compiler build tasks and targets files.

Fixes #9640.

* Update DSR.PortablePdb to 60818-04

* Update Dependencies.props

Use nuget restore to determine the exact dependencies we need to insert into VS.