"Roslyn" -> "Microsoft" or "Microsoft.CodeAnalysis"

Plumb the features flag through our MSBuild and Workspaces layers.

closes #3879

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.

Fell out of our normal cadence for this during the push for RTM. Now that we
had a bit more breathing room getting us back on track here.

These two changes should have been included in
https://github.com/dotnet/roslyn/commit/8c9ed547c5bc6ad5a9842945688da9b2f189ac8c
but,

1. The references didn't match any of the extraneous VSIX names because
they inline variables.
2. OpenSourceDebug.csproj doesn't drop a VSIX in the output folder, so it
was missed by my validation pass.

The net effect is to remove Microsoft.Build.Tasks.Core.dll,
Microsoft.Build.Utilities.Core.dll, and
Microsoft.VisualStudio.Shell.14.0.dll from OpenSourceDebug.vsix.

Mechanically apply <Private>False</Private> to the newly-appeared
references.

If we have a solution with circular references, leave one as a metadata
reference to prevent deadlocks when trying to get compilations.

The `FilePathUtilities.GetRelativePath(...)` method is used in various
places in the Workspaces and VS layers to generate a relative path from
a starting directory to a target file or directory. However, it has a
bug where it will generate an incorrect path if you need to go up more
than one level from the starting directory to get to the target.

For example, if the starting directory is `C:\Alpha\Beta\Gamma` and the
target is `C:\Alpha\SomeFile.txt`, the correct relative path would be
`..\..\SomeFile.txt`. Instead, we get `..\..\..\SomeFile.txt` (note the
extra `..`).

This commit fixes the issue and adds unit tests for this method.

Fixes #1146

This deletes the empty and unused Desktop assemblies from the code base.

This change removes all C# specific references from
MS.CA.Workspaces.CSharp.Desktop.dll.  This demonstrates the pattern by
which we could migrate the language specific MSBuild code into a shared
DLL which has no language specific dependencies.  This pattern will
allow us to delete both MS.CA.W.CSHarpDesktop and
MS.Ca.W.VisualBasic.Desktop.

This removes the majority of references to the deprecated Desktop
assemblies.  The only remaining ones are in the VSIX projects.  They
must remain until we actually delete the assemblies.

This moves the contents of the CSharp and Visual Basic Desktop
Projects to the Portable counterparts.

closes #2587

Be tolerant of msbuild failures when opening project files to find output path for loading metadata instead of project sources.