Fix relative path handling on Windows.

On Windows, `PathUtility.GetRelativePath` was not properly handling
paths that differed by case in the drive reference (e.g. "C:\" vs.
"c:\").  The fix was to add the missing case-insensitive comparison
argument.

Replaced uses of `PathUtility.GetRelativePath` with
`Path.GetRelativePath` where possible (requires 2.0.0+).

Additionally, `PathUtility.RemoveExtraPathSeparators` was not handling
paths with drive references on Windows.  If the path contained a drive
reference, the separator between the drive reference and the first part
of the path was removed.  This is due to `Path.Combine` not handling
this case, so an explicit concatenation of the separator was added.

This commit resolves issue #7699.

src/dotnet/MsbuildProject.cs

@@ -256,7 +256,7 @@ namespace Microsoft.DotNet.Tools
 
             string fullPath = Path.GetFullPath(reference);
             ret.Add(fullPath);
-            ret.Add(PathUtility.GetRelativePath(ProjectDirectory, fullPath));
+            ret.Add(Path.GetRelativePath(ProjectDirectory, fullPath));
 
             return ret;
         }