The synchronous API still exists for TS to use until they
move over to the entirely async api.

In order to gather more information on
https://devdiv.visualstudio.com/DevDiv/_workitems?id=209299, we created
https://github.com/dotnet/roslyn/pull/16918 to capture the text of the
linked documents that aren't in sync. However, we tried to do so by
putting their text in local variables that were optimized away in dumps.

This change introduces a custom exception object that holds onto the
text of the conflicting documents and reports that exception.

Related to #9960

Ensure that all *.*proj, *.props, and *.targets files are prefixed with the xml and copyright elements

This setting controls binary deployment hence it needs to be centrally
managed to help ensure we have a correct build.  Custom projects can
continue to control this setting themselves.

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.

Previously, we'd raise workspace events on whatever synchronization
context created the workspace. If you created the VisualStudioWorkspace
on a background thread, then this would mean we'd raise events there
which is not expected by many parts of our codebase. This corrects that
ensuring they're always raised where they should be. This also applies
to all workspaces in Visual Studio, which has been a source of bugs
when somebody had another workspace that they created themselves
which broke expectations.

While doing this I simplifed the APIs to clarify what they were being
used for and also removed some FactoryFactories that were unnecessary.