fix!: Fix resource dictionary path handling

BREAKING CHANGE: Source paths starting with '/' are now considered relative to project root instead of current file.

src/Uno.UI.Tests/Windows_UI_Xaml/Given_ResourceDictionary.cs

@@ -712,6 +712,16 @@ public void When_Relative_Path_With_Leading_Slash_From_Root()
 			Assert.AreEqual(withoutSlash, withSlash);
 		}
 
+		[TestMethod]
+		public void When_Relative_Path_With_Leading_Slash_From_Non_Root()
+		{
+			var withSlash = XamlFilePathHelper.ResolveAbsoluteSource("Dictionaries/App.xaml", "/App/Xaml/Test_Dictionary.xaml");
+			var withoutSlash = XamlFilePathHelper.ResolveAbsoluteSource("Dictionaries/App.xaml", "App/Xaml/Test_Dictionary.xaml");
+
+			Assert.AreEqual("App/Xaml/Test_Dictionary.xaml", withSlash);
+			Assert.AreEqual("Dictionaries/App/Xaml/Test_Dictionary.xaml", withoutSlash);
+		}
+
 		[TestMethod]
 		public void When_SharedHelpers_FindResource()
 		{

src/Uno.UI/UI/Xaml/XamlFilePathHelper.shared.cs

@@ -27,9 +27,10 @@ internal static string ResolveAbsoluteSource(string origin, string relativeTarge
 				var trimmedPath = relativeTargetPath.TrimStart(AppXIdentifier);
 				return trimmedPath;
 			}
-
-			// Strip leading forward-slash, if any, to match the path the target file is indexed by
-			relativeTargetPath = relativeTargetPath.TrimStart(new[] { '/' });
+			else if (relativeTargetPath.StartsWith("/", StringComparison.Ordinal))
+			{
+				return relativeTargetPath.Substring(1);
+			}
 
 			var originDirectory = Path.GetDirectoryName(origin);
 			if (originDirectory.IsNullOrWhiteSpace())