When multiple VS2017 instances exist, OmniSharp can select one without Roslyn

[kzu]

The current implementation only checks that the installation is complete. It's possible, however, to also check for an installation that has the required Microsoft.VisualStudio.Component.Roslyn.Compiler component (see https://docs.microsoft.com/en-us/visualstudio/install/workload-component-id-vs-community).

PR to follow.

To test the failure, install a new VS2017 instance and select just the Core Editor component. When restarting VS Code and opening a solution file (or directory), OmniSharp will pick up the latest installation, which won't have the roslyn compilers and therefore cause the following exception on load:

[info]: OmniSharp.MSBuild.MSBuildProjectSystem
        Loading project: c:\Code\Personal\moq\src\Proxy\Proxy\Moq.Proxy.csproj
[warn]: OmniSharp.MSBuild.MSBuildProjectSystem
        Failed to load project file 'c:\Code\Personal\moq\src\Proxy\Proxy\Moq.Proxy.csproj'.
c:\Code\Personal\moq\src\Proxy\Proxy\Moq.Proxy.csproj(1,1)
Microsoft.Build.Exceptions.InvalidProjectFileException: The imported project "C:\Program Files (x86)\Microsoft Visual Studio\d15rel\Enterprise\MSBuild\15.0\Bin\Roslyn\Microsoft.CSharp.Core.targets" was not found. Confirm that the path in the <Import> declaration is correct, and that the file exists on disk.  C:\Program Files (x86)\Microsoft Visual Studio\d15rel\Enterprise\MSBuild\15.0\Bin\Microsoft.CSharp.CurrentVersion.targets
   at Microsoft.Build.Shared.ProjectErrorUtilities.ThrowInvalidProject(String errorSubCategoryResourceName, IElementLocation elementLocation, String resourceName, Object[] args)
   at Microsoft.Build.Shared.ProjectErrorUtilities.ThrowInvalidProject[T1](IElementLocation elementLocation, String resourceName, T1 arg0)
   at Microsoft.Build.Evaluation.Evaluator`4.ExpandAndLoadImportsFromUnescapedImportExpression(String directoryOfImportingFile, ProjectImportElement importElement, String unescapedExpression, Boolean throwOnFileNotExistsError, List`1& imports)
   at Microsoft.Build.Evaluation.Evaluator`4.ExpandAndLoadImportsFromUnescapedImportExpressionConditioned(String directoryOfImportingFile, ProjectImportElement importElement, List`1& projects, Boolean throwOnFileNotExistsError)
   at Microsoft.Build.Evaluation.Evaluator`4.ExpandAndLoadImports(String directoryOfImportingFile, ProjectImportElement importElement)
   at Microsoft.Build.Evaluation.Evaluator`4.EvaluateImportElement(String directoryOfImportingFile, ProjectImportElement importElement)
   at Microsoft.Build.Evaluation.Evaluator`4.PerformDepthFirstPass(ProjectRootElement currentProjectOrImport)
   at Microsoft.Build.Evaluation.Evaluator`4.EvaluateImportElement(String directoryOfImportingFile, ProjectImportElement importElement)
   at Microsoft.Build.Evaluation.Evaluator`4.PerformDepthFirstPass(ProjectRootElement currentProjectOrImport)
   at Microsoft.Build.Evaluation.Evaluator`4.EvaluateImportElement(String directoryOfImportingFile, ProjectImportElement importElement)
   at Microsoft.Build.Evaluation.Evaluator`4.PerformDepthFirstPass(ProjectRootElement currentProjectOrImport)
   at Microsoft.Build.Evaluation.Evaluator`4.EvaluateImportElement(String directoryOfImportingFile, ProjectImportElement importElement)
   at Microsoft.Build.Evaluation.Evaluator`4.PerformDepthFirstPass(ProjectRootElement currentProjectOrImport)
   at Microsoft.Build.Evaluation.Evaluator`4.Evaluate(ILoggingService loggingService, BuildEventContext buildEventContext)
   at Microsoft.Build.Evaluation.Project.Reevaluate(ILoggingService loggingServiceForEvaluation, ProjectLoadSettings loadSettings)
   at Microsoft.Build.Evaluation.Project.ReevaluateIfNecessary(ILoggingService loggingServiceForEvaluation, ProjectLoadSettings loadSettings)
   at Microsoft.Build.Evaluation.Project.Initialize(IDictionary`2 globalProperties, String toolsVersion, String subToolsetVersion, ProjectLoadSettings loadSettings)
   at Microsoft.Build.Evaluation.Project..ctor(String projectFile, IDictionary`2 globalProperties, String toolsVersion, String subToolsetVersion, ProjectCollection projectCollection, ProjectLoadSettings loadSettings)
   at Microsoft.Build.Evaluation.ProjectCollection.LoadProject(String fileName, IDictionary`2 globalProperties, String toolsVersion)
   at OmniSharp.MSBuild.ProjectFile.ProjectFileInfo.LoadProject(String filePath, String solutionDirectory, ILogger logger, MSBuildInstance msbuildInstance, SdksPathResolver sdksPathResolver, MSBuildOptions options, ICollection`1 diagnostics, ImmutableArray`1& targetFrameworks) in C:\projects\omnisharp-roslyn\src\OmniSharp.MSBuild\ProjectFile\ProjectFileInfo.cs:line 111
   at OmniSharp.MSBuild.ProjectFile.ProjectFileInfo.Create(String filePath, String solutionDirectory, ILogger logger, MSBuildInstance msbuildInstance, SdksPathResolver sdksPathResolver, MSBuildOptions options, ICollection`1 diagnostics) in C:\projects\omnisharp-roslyn\src\OmniSharp.MSBuild\ProjectFile\ProjectFileInfo.cs:line 81
   at OmniSharp.MSBuild.MSBuildProjectSystem.LoadProject(String projectFilePath) in C:\projects\omnisharp-roslyn\src\OmniSharp.MSBuild\MSBuildProjectSystem.cs:line 321