Fix imported link paths not being nested correctly in the MGCB browser. (MonoGame#8739)

XNA contentproj files use Windows paths separated by backslashes. MGCB
expects forward slashes to be able to nest content items correctly.

The call to PathHelper.GetRelativePath() below the lines changed use
System.Uri methods, which has the effect of performing backslash ->
slash conversions on the supplied source file path, so this seemed the
correct location to convert the link path too.

Without this change, any contentproj that references items with a
`<Link>` path will be shown at the root of the MGCB project. With this
change, the items will be nested at the correct location in the browser.

---------

Co-authored-by: Simon (Darkside) Jackson <[email protected]>

Author: ben-marsh

Tools/MonoGame.Content.Builder.Editor/Common/PipelineProjectParser.cs

@@ -175,7 +175,7 @@ public bool AddContent(string sourceFile, bool skipDuplicates)
                 sourceFile = split[0];
 
                 if (split.Length > 0)
-                    link = split[1];
+                    link = PathHelper.Normalize(split[1]);
             }
 
             // Make sure the source file is relative to the project.
@@ -230,7 +230,7 @@ public void OnCopy(string sourceFile)
                 sourceFile = split[0];
 
                 if (split.Length > 0)
-                    link = split[1];
+                    link = PathHelper.Normalize(split[1]);
             }
 
             // Make sure the source file is relative to the project.