Convert solution relative paths to OS slashes

Fixes dotnet#2022, fixes dotnet#1957.

Ported the code change only from mono@86adc96.

src/Build/Construction/Solution/ProjectInSolution.cs

@@ -8,13 +8,13 @@
 using System.Security;
 using System.Text;
 using System.Xml;
+using Microsoft.Build.Shared;
 
 using XMakeAttributes = Microsoft.Build.Shared.XMakeAttributes;
 using ProjectFileErrorUtilities = Microsoft.Build.Shared.ProjectFileErrorUtilities;
 using BuildEventFileInfo = Microsoft.Build.Shared.BuildEventFileInfo;
 using ErrorUtilities = Microsoft.Build.Shared.ErrorUtilities;
 using System.Collections.ObjectModel;
-using System.Linq;
 
 namespace Microsoft.Build.Construction
 {
@@ -155,7 +155,11 @@ public string ProjectName
         public string RelativePath
         {
             get { return _relativePath; }
-            internal set { _relativePath = value; }
+            internal set
+            {
+                _relativePath = FileUtilities.MaybeAdjustFilePath(value,
+                                                    baseDirectory:this.ParentSolution.SolutionFileDirectory ?? String.Empty);
+            }
         }
 
         /// <summary>