Fix #911 handle perforce files (#400)

- Additional handling of document events with "perforce:" or other SCM strings
- Much of the handling had been added already, but adding in additional validation and ensuring that everywhere `Workspace.GetFile` is called we add a not-null check to avoid null reference errors

src/Microsoft.SqlTools.ServiceLayer/Utility/FileUtils.cs

@@ -4,7 +4,7 @@
 //
 using System;
 using System.IO;
-namespace Microsoft.SqlTools.ServiceLayer.QueryExecution
+namespace Microsoft.SqlTools.ServiceLayer.Utility
 {
     internal static class FileUtilities
     {
@@ -136,8 +136,18 @@ namespace Microsoft.SqlTools.ServiceLayer.QueryExecution
                 File.SetAttributes(fullFilePath, FileAttributes.Normal);
             }
         }
+
+        internal static ResolvedFile TryGetFullPath(string filePath)
+        {
+            try
+            {
+                return new ResolvedFile(Path.GetFullPath(filePath), true);
+            }
+            catch(NotSupportedException)
+            {
+                // This is not a standard path. 
+                return new ResolvedFile(filePath, false);
+            }
+        }
     }
-
-
-    
 }