Memoize calls to ProjectAlreadyReferencesProject where possible to accelerate loading of projects with larger project-reference graphs.

Also, use the right solution load event so we don't have to delay.

Finally, use the IVsSolution APIs to get project file names and guids.

* Enable building and testing on machines with only Dev15 Preview 3 installed

1. Auto-detect the VS common tools path
2. Convert analyzer samples to import Roslyn targets
3. Ensure dependencies are copied to the output directory
4. Workaround: Use local Microsoft.Net.Compilers.props until we move to a compiler toolset with https://github.com/dotnet/roslyn/commit/05c12ebfcdd08a02dbceded5327a8da7a7df23be
5. Replace references to Microsoft.VisualStudio.ComponentModelHost.dll (VisualStudioReferenceAssemblyVersion) with a nuget reference to RoslynDependencies.Microsoft.VisualStudio.ComponentModelHost
6. Add nuget references to Roslyn.Microsoft.Build, Roslyn.Microsoft.VisualStudio.ExtensionManager, RoslynDependencies.Microsoft.VisualStudio.Diagnostics.PerformanceProvider and RoslynDependencies.Microsoft.VisualStudio.CodeAnalysis.Sdk.UI.

* Remove workaround now that new NuGet packages have been uploaded to nuget.org

* Remove preprocessor directive MSBUILD12 and its references

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

Fixes #1146

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

The API Environment.CurrentDirectory is not available on CoreFX.
Switching all uses to Directory.GetCurrentDirectory which is an
equivalent API (it is actually how Environment.CurrentDirectory is often
implemented).

closes #1829