These projects reference the compiler EXE projects and hence copy them
to the output directory. This appears to not cause the correct
app.config file (ex csc.exe.config) to be copied along with it. It's
missing the necessary binding redirects that are generated from a direct
compile of the project. As a result when it' invoked from the unit tests
it fails.

It's possible this is a SDK bug and I will dig into that. For now though
I need to unblock the suites.

The continued mixing of assets between net46 and net461 was causing a
huge number of binding redirect issues in our test assets. Need to move
everything to a consistent 461 to avoid this.

This lets us avoid hard coding in version numbers for the core tools
we use in our build.

This removes the hard coded version number for dotnet-xunit. It now uses awk
to parse out the actual version in Packages.props.

Now that we are targeting 4.6.1 there is a new redirect we need to add
to our unit tests.

This changes the unix legs to use the 2.0 SDK. They are likely to suffer from the
same problem as the windows CoreClr leg in that they aren't producing XML files. But
we should be able to fix both of those at the same time.

The compiler tools needed to be targetting netcoreapp2.0 in order to
produce runnable executables.

This is at least temporarily using `dotnet vstest` to execute the tests.
That allows us to run the tests but can't do things like generate
compatible XML files that Jenkins can process.

The new structure of the tests does mean that our Linux / Mac jobs are
now broken though since we no longer have the deploy project.

This primarily moves us to the 2.2.0 version of xunit. In order to make
that transition though a number of other changes needed to happen as
well:

1. Needed to move our desktop target to 4.6.1. This should've been
required some time ago but we were essentially depending on bugs in the
SDK / NuGet.
1. Remove the DeployCoreClrTestRuntime project. This entire idea
depended on having the xunit.console.netcore package. This has since
been deleted and doesn't work with the 2.0 SDK.
1. Remove the deploy compiler tools project. Can just use dotnet exec
now and that's much more efficient.
1. Resolve a new ambiguity with KeyValuePair. A helper of ours now
exists in netcoreapp2.0.

I fully expect a number of Jenkins legs to fail with this change: build
correctness, coreclr and ubuntu. But I want to validate our other legs
and am still trackind down answers needed to fix the others.

Changes:

- Move to new MSBuild toolset that supports resolvers.
- Handle the case where %PATH% contains an empty entry.

Changes our core build scripts to download the CLI / SDK as needed.

Now that we are not tied to roslyn-internal anymore it's possible to
simplify this action a bit:

- Remove the separate script as it's not necessary and can be easily
inlined
- Remove the copy to the root of the source tree. Not necessary anymore
as all our scripts use the returned variable.

Extract code into helper method.

* Add and implement IScriptEnvironmentService

* Remove unnecessary export

Better error reporting for VsLanguageDebugInfo

Fixed IDE0009 suggestion in nameof expression used to initialize static member

Enable publishing of features/readonly-ref artefacts.

Formation completion items after commit if they ask for that.

Properly collapse the right range when encountering nested #if regions.

Be resilient to getting busy messages back from sqlite while retrieving data.