The dependency projects were added as a way to help unify our project.json references.  Instead of every project referencing the NuGet package, we had a dependency project which referenced it and everyone referenced that dependency project.  Due to the transitive nature of NuGet it would be roughly equivalent.

This system had a number of downsides:

1. There was no enforcement.  Even though dependency projects existed, there was no mechanism to force developers to use them.
2. Getting the granularity correct was tricky.
3. The projects themselves have special props / targets to get them to produce no output.  This can trip up MSBuild up to date checks.

Since then though we've begun using RepoUtil to verify our NuGet reference correctness.  This removed the need for these projects and hence I'm removing them from our build.

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.

The dependency projects serve a few purposes:

- Help ensure package unity by serving as a single place for a NuGet reference.
- Make NuGet updates simple by having a single place to change.
- Grouping together related packages to make it easy to take a dependency on say Visual Studio editor.

The first two points are largely obsolete now that our project.json references are strictly validated on every build and have a single update mechanism.  This meant a lot of our depnedency projects were just making Roslyn.sln bigger.

As such I went through and deleted all of them which had only 1 or 2 project.json entries.  I left the VS ones which tend to group 10+ references together.  They are still serving a valid "reference VS" purpose.

This reverts commit 31377ce7.

Update TargetFrameworkVersions.

Remove unneeded app.config files from samples test projects.
Add missing xunit toolset imports to samples test projects.

Fixes #9707.

Ensured that all uses of app.config in our product / test code base is
used in the lower case form.  This helps with our Unix efforts where
mixing the case can be significant.  Updated all of the project
references to ensure they used the lower case version as well.

Updating System.Reflection.Metadata to 1.1.0-alpha-0015 and System.Collections.Immutable to 1.1.37.0

The targets files were previously split into an open version in
Microsoft.CodeAnalysis.Toolset.Open and a closed version in build/. This
unifies the four targets into two new targets, VSL.Settings.targets and
VSL.Imports.targets, and moves all the targets from ..Toolset.Open into
the build/ folder.

Upgrades System.Collections.Immutable to 1.1.34-rc and
System.Reflection.Metadata to 1.0.19-rc.

This reverts commit b37052d0.

This moves our settings / targets files into the Open directory.  These files are a referenced by virtually all of the projects in the Open directory and are a necessary part of the build process.  Making this change is a necessary step in removing the project scrubber.  (changeset 1396832)

This changes our projects to consume xUnit through NuGet instead of our checked in binaries in Closed\Tools\xUnit.  This is another step in removing project scrubber.  (changeset 1396548)

This changeset introduces the concept of a "global" nuget package. The references to the NuGet packages are inserted into the Open VSL.Settings.targets file, which is imported into every project in Roslyn. This ensures that every project automatically gets these "global" packages. (changeset 1386548)