Default to RoslynToolchain

[timcassell]

Fixes #2405

[adamsitnik]

@ViktorHofer @ericstj do you know if it's possible to determine whether the Full .NET Framework exe that is currently being executed has been compiled using the classic project file (using packages.config etc)?

Context: when BDN needs to create and build a new exe we would prefer to use the modern project files.

[timcassell]

@adamsitnik The changes here basically just uses RoslynToolchain if there are no build settings changes, otherwise it will use CsProjClassicNetToolchain which will still fail with packages.config. It just fixes the default case (what first-time users will see).

I already have a working prototype that fixes using packages.config (along with a few other issues), but it relies on changes in #2393.

[ViktorHofer]

@ViktorHofer @ericstj do you know if it's possible to determine whether the Full .NET Framework exe that is currently being executed has been compiled using the classic project file (using packages.config etc)?
Context: when BDN needs to create and build a new exe we would prefer to use the modern project files.

Unsure but I would surprised if so. The underlying build infrastructure (projejct-system format) shouldn't leak into the app's output.

[AndreyAkinshin]

@timcassell thanks!