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);
+            }
+        }
     }
-
-
-    
 }

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

@@ -0,0 +1,39 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+
+namespace Microsoft.SqlTools.ServiceLayer.Utility
+{
+    /// <summary>
+    /// Utility object holding a result of a file resolve action.
+    ///
+    /// Workspace APIs support reading from disk if a file hasn't been
+    /// officially opened via VSCode APIs with a buffer. This is problematic
+    /// in the case where it's not a file on disk as any attempt will cause
+    /// an exception to be thrown.
+    /// 
+    /// To mitigate this a ResolvedFile object has an additional flag indicating
+    /// if the file can be read from disk.
+    /// </summary>
+    internal class ResolvedFile
+    {
+        public ResolvedFile(string filePath, bool canReadFromDisk)
+        {
+            FilePath = filePath;
+            CanReadFromDisk = canReadFromDisk;
+        }
+
+        public string FilePath { get; private set; }
+        public bool CanReadFromDisk { get; private set; }
+
+        public string LowercaseFilePath
+        {
+            get
+            {
+                return FilePath?.ToLower();
+            }
+        }
+    }
+}