Remove check for path starting with root, assume path is always relative to the root

src/Umbraco.Web.Common/AspNetCore/AspNetCoreHostingEnvironment.cs

@@ -85,7 +85,15 @@ namespace Umbraco.Web.Common.AspNetCore
         private string MapPath(string root, string path)
         {
             var newPath = path.Replace('/', Path.DirectorySeparatorChar).Replace('\\', Path.DirectorySeparatorChar);
-            return newPath.StartsWith(root) ? newPath : Path.Combine(root, newPath.TrimStart('~', '/'));
+
+            // TODO: This is a temporary error because we switched from IOHelper.MapPath to HostingEnvironment.MapPathXXX
+            // IOHelper would check if the path passed in started with the root, and not prepend the root again if it did,
+            // however if you are requesting a path be mapped, it should always assume the path is relative to the root, not
+            // absolute in the file system.  This error will help us find and fix improper uses, and should be removed once
+            // all those uses have been found and fixed
+            if (newPath.StartsWith(root)) throw new ArgumentException("The path appears to already be fully qualified.  Please remove the call to MapPath");
+
+            return Path.Combine(root, newPath.TrimStart('~', '/'));
         }
 
         public string ToAbsolute(string virtualPath)