Implicit framework references were disabled at the time we moved over to
the new SDK due to some remanants of project.json in our build. Those
have all been cleaned up now and hence moving us over to using implicit
framework references.

Not having this enabled meant the API surface we were targeting in our
projects was not actually what was available in the project target
framework. Instead it was the intersection of the target framework and
the set of NuGet packages we reference. This meant that code which
should work simply wasn't due to the API surface not being correct.

This moves a few of the patches from sourcebuild into the Roslyn source proper.
We now condition on DotNetBuildFromSource in some of our imports, and ensure
that we're not targeting netframework when building for it. We also add a sln
for SourceBuild, SourceBuild.sln, which includes the minimum projects that it
needs.

This change makes the legacy behavior contingent on the runtime of the
compiler. When the compiler is running on the desktop runtime, the
legacy signing code will be used. When running on CoreCLR, the new
portable signing code will be used. This fixes key container signing for
almost all scenarios (signing using a key container when the compiler is
running on the desktop framework). Signing using a key container on
Windows using a compiler running on CoreCLR is still broken, but that is
a far rarer case.

This change also fixes delaysign, which would full sign the file if the
keyfile passed to the compiler contains a private key.

Fixes #25340
Fixes #25424

Prefer to use runtime checks instead.

This changes the GC used by the compiler binaries to be the Server GC when
running on CoreClr. This is already the GC we use when running on desktop.

In the new SDK we no longer have a need for ProjectGuid. This change
will:

- Remove ProjectGuid from project files
- Remove Name + Project items from ProjectReference entries
- Change BuildBoss to enforce both of these items

Migrated the remainder of our code base to use the new SDK via the
ConvertPackageRef tool.

https://github.com/jaredpar/ConvertPackageRef

This is an automated migration of our project files to the new SDK /
PackageReference.  It is done using the tool located here:

https://github.com/jaredpar/ConvertPackageRef

This should either be defined globally or not at all.  Repeating all the logic on a local level is just wasted time.

TLDR: artifacts are going to move around in Binaries\Debug and the directory is going to get a lot bigger.

At a high level build projects can be classified into three categories based on how they write output:

- incorrect: a given output path is written to more than once with different contents
- less correct: a given output path is written to more than once but always with the same content
- correct: a given output path is written to exactly once

Today the roslyn build is decidedly “incorrect” as pretty much every file is written directly into Binaries\Debug. This means it ends up writing pretty much every Visual Studio SDK DLL twice: once for Dev14 and once for Dev15. For example at various points in the build Binaries\Debug\Microsoft.VisualStudio.Text.Data.dll may refer to Dev14 and at others it’s Dev15. If this seems like a scary proposition for a build that’s because it is indeed scary and it has real consequences. By now pretty much everyone on the team has hit the build race condition that is dragging down our PRs.

The general fix here is to move build outputs into separate directories. Instead of building to $(Configuration) projects now build into say $(Configuration)\Exes\$(MSBuildProjectFileName). This will have a substantial increase in the size of Binaries. We will be looking into ways to reduce that. In the short term though build stability far outweighs the size increase.

This change takes us most of the way to "correct". There are several places I had to compromise in order to get this initial change in:

- UnitTests still build to a common output folder (one for Dev14, another for Dev15). Pulling unit tests apart is going to take a bit of work.
- Every project has a <RoslynProjectType> entry. This will go away in the future for most projects. It's temporarily needed so I can fix roslyn-internal in parallel without taking down the build.
- VSL.Imports.targets is messy. Unavoidable for now due to the above. It will get cleaner as I iterate on this.

None of these are relevant to the underlying race condition. Hence it's okay to push them off.

The ImportGroup element is just noise.  It was also used very inconsistently in the repo and often within the same project file.  Just remove it.

This removes the hard-coded binding redirects that exist within our
source tree in favor of redirects that are computed automatically. This
takes advantage of the built-in binding redirect generation support
in MSBuild. In order to get the redirects to merge properly with
the static stuff we have, I renamed the .exe.config files to App.config
which is the name the tooling looks for.

Addresses #7612.

This reverts commit 31377ce7.

Update TargetFrameworkVersions.