fix collecting unexpected solution file like 'sln*'

[itn3000]

Overview

This PR fixes omnisharp crash when the directory has '*.slnxxx' file.

Problem Detail

Environment

• windows8.1-x64
• omnisharp-vscode: 1.16.2
• vscode: 1.28.2

Steps to reproduce

1. create new console project in empty directory
2. create 'x.slnproj' and edit <Project Sdk="MSBuild.SolutionSdk/0.1.0"></Project>
   • This is MSBuild.SolutionSdk format
3. Restart Omnisharp

Expected behavior

Omnisharp start to watch the project folder.

Actual Behavior

Omnisharp throw the following exception and stop to watch the project.

OmniSharp.MSBuild.SolutionParsing.InvalidSolutionFileException: Solution header should be on first or second line.
   場所 OmniSharp.MSBuild.SolutionParsing.SolutionFile.ParseHeaderAndVersion(Scanner scanner)
   場所 OmniSharp.MSBuild.SolutionParsing.SolutionFile.Parse(String text)
   場所 OmniSharp.MSBuild.ProjectSystem.GetProjectPathsFromSolution(String solutionFilePath)
   場所 OmniSharp.MSBuild.ProjectSystem.Initalize(IConfiguration configuration)
   場所 OmniSharp.WorkspaceInitializer.Initialize(IServiceProvider serviceProvider, CompositionHost compositionHost)

This behavior is caused by Directory.GetFiles specification (in 'Note' section)

[DustinCampbell]

LGTM. Thanks!

[filipw]

makes sense indeed