[xamlc] stop using pack:// with System.Uri in .NET 7 (#7859)

This reverts the XamlC changes in b8646cb.

Originally we were hitting:

    XamlApp.xaml Invalid URI: The hostname could not be parsed. [D:\a\_work\1\s\src\Controls\samples\Controls.Sample\Maui.Controls.Sample.csproj]
    at System.Uri.CreateThis(String uri, Boolean dontEscape, UriKind uriKind, UriCreationOptions& creationOptions)
    at System.Uri..ctor(String uriString, UriKind uriKind)
    at Microsoft.Maui.Controls.ResourceDictionary.RDSourceTypeConverter.GetResourcePath(Uri uri, String rootTargetPath)
    at Microsoft.Maui.Controls.XamlC.RDSourceTypeConverter.ConvertFromString(String value, ILContext context, BaseNode node)+MoveNext()
    at Microsoft.Maui.Controls.Build.Tasks.NodeILExtensions.PushConvertedValue(ValueNode node, ILContext context, TypeReference targetTypeRef, TypeReference typeConverter, IEnumerable`1 pushServiceProvider, Boolean boxValueTypes, Boolean unboxValueTypes)+MoveNext()
    at Microsoft.Maui.Controls.Build.Tasks.SetPropertiesVisitor.Set(VariableDefinition parent, String localName, INode node, IXmlLineInfo iXmlLineInfo, ILContext context)+MoveNext()
    at Microsoft.Maui.Controls.Build.Tasks.ILProcessorExtensions.Append(ILProcessor processor, IEnumerable`1 instructions)
    at Microsoft.Maui.Controls.Build.Tasks.SetPropertiesVisitor.Visit(ValueNode node, INode parentNode)
    at Microsoft.Maui.Controls.Xaml.ElementNode.Accept(IXamlNodeVisitor visitor, INode parentNode)
    at Microsoft.Maui.Controls.Build.Tasks.SetResourcesVisitor.Visit(ElementNode node, INode parentNode)
    at Microsoft.Maui.Controls.Xaml.ElementNode.Accept(IXamlNodeVisitor visitor, INode parentNode)
    at Microsoft.Maui.Controls.Xaml.ElementNode.Accept(IXamlNodeVisitor visitor, INode parentNode)
    at Microsoft.Maui.Controls.Xaml.RootNode.Accept(IXamlNodeVisitor visitor, INode parentNode)
    at Microsoft.Maui.Controls.Build.Tasks.XamlCTask.TryCoreCompile(MethodDefinition initComp, ILRootNode rootnode, String xamlFilePath, Exception& exception)

I added some logging around this location, and the PR build was
green... So we merged, but then the build on the `net7.0` branch
failed with:

    XamlApp.xaml(11,37): error XFC0128: GetResourcePath failed: uri=AppResources.xaml, rootTargetPath=XamlApp.xaml

What is weird is a test like this works fine!

    [Test]
    public void GetResourcePath()
    {
        var result = ResourceDictionary.RDSourceTypeConverter.GetResourcePath(new Uri("AppResources.xaml", UriKind.Relative), "XamlApp.xaml");
        Assert.IsNotNull(result);
        Assert.IsNotEmpty(result);
    }

So this must be something that doesn't fail every time. I was able to
reproduce it locally *once*, but then not again...

After some discussion in: dotnet/runtime#70443 (comment)

It appears that usage of `pack://` is *sometimes* picking up a custom
`UriParser`, such as:

https://github.com/dotnet/runtime/blob/eef6c7f6cb7f73163313d349fbb796f37b4c3996/src/libraries/System.IO.Packaging/src/System/IO/Packaging/PackUriHelper.PackUriScheme.cs#L226-L229

If we just use something else, like `maui://`, I believe it will work
around this issue. I'll leave it up to the MAUI team if they want to
rewrite this method using `System.IO.Path` in the future.

src/Controls/src/Build.Tasks/BuildException.cs

@@ -89,7 +89,6 @@ class BuildExceptionCode
 		public static BuildExceptionCode ResourceDictDuplicateKey = new BuildExceptionCode("XFC0125", nameof(ResourceDictDuplicateKey), "");
 		public static BuildExceptionCode ResourceDictMissingKey = new BuildExceptionCode("XFC0126", nameof(ResourceDictMissingKey), "");
 		public static BuildExceptionCode XKeyNotLiteral = new BuildExceptionCode("XFC0127", nameof(XKeyNotLiteral), "");
-		public static BuildExceptionCode GetResourcePath = new BuildExceptionCode("XFC0128", nameof(GetResourcePath), "");
 
 		public string Code { get; private set; }
 		public string ErrorMessageKey { get; private set; }

src/Controls/src/Build.Tasks/CompiledConverters/RDSourceTypeConverter.cs

@@ -40,17 +40,9 @@ public IEnumerable<Instruction> ConvertFromString(string value, ILContext contex
 					module = currentModule.AssemblyResolver.Resolve(ar).MainModule;
 				}
 			}
-
 			var uri = new Uri(value, UriKind.Relative);
-			string resourcePath;
-			try
-			{
-				resourcePath = ResourceDictionary.RDSourceTypeConverter.GetResourcePath(uri, rootTargetPath);
-			}
-			catch (Exception exc)
-			{
-				throw new BuildException(BuildExceptionCode.GetResourcePath, node, exc, uri, rootTargetPath);
-			}
+
+			var resourcePath = ResourceDictionary.RDSourceTypeConverter.GetResourcePath(uri, rootTargetPath);
 
 			//fail early
 			var resourceId = XamlCTask.GetResourceIdForPath(module, resourcePath);

src/Controls/src/Build.Tasks/ErrorMessages.resx

@@ -248,8 +248,5 @@
   <data name="XKeyNotLiteral" xml:space="preserve">
     <value>x:Key expects a string literal.</value>
   </data>
-  <data name="GetResourcePath" xml:space="preserve">
-    <value>GetResourcePath failed: uri={0}, rootTargetPath={1}</value>
-  </data>
 
 </root>

src/Controls/src/Core/ResourceDictionary.cs

@@ -17,6 +17,7 @@ namespace Microsoft.Maui.Controls
 	/// <include file="../../docs/Microsoft.Maui.Controls/ResourceDictionary.xml" path="Type[@FullName='Microsoft.Maui.Controls.ResourceDictionary']/Docs" />
 	public class ResourceDictionary : IResourceDictionary, IDictionary<string, object>
 	{
+		const string GetResourcePathUriScheme = "maui://";
 		static ConditionalWeakTable<Type, ResourceDictionary> s_instances = new ConditionalWeakTable<Type, ResourceDictionary>();
 		readonly Dictionary<string, object> _innerDictionary = new Dictionary<string, object>();
 		ResourceDictionary _mergedInstance;
@@ -383,10 +384,11 @@ object IExtendedTypeConverter.ConvertFromInvariantString(string value, IServiceP
 
 			internal static string GetResourcePath(Uri uri, string rootTargetPath)
 			{
-				//need a fake scheme so it's not seen as file:// uri, and the forward slashes are valid on all plats
+				// GetResourcePathUriScheme is a fake scheme so it's not seen as file:// uri,
+				// and the forward slashes are valid on all plats
 				var resourceUri = uri.OriginalString.StartsWith("/", StringComparison.Ordinal)
-									 ? new Uri($"pack://{uri.OriginalString}", UriKind.Absolute)
-									 : new Uri($"pack:///{rootTargetPath}/../{uri.OriginalString}", UriKind.Absolute);
+									 ? new Uri($"{GetResourcePathUriScheme}{uri.OriginalString}", UriKind.Absolute)
+									 : new Uri($"{GetResourcePathUriScheme}/{rootTargetPath}/../{uri.OriginalString}", UriKind.Absolute);
 
 				//drop the leading '/'
 				return resourceUri.AbsolutePath.Substring(1);