Simplify default source type retrieval in PackageManager

- Integrate the fix from "Fix for Source Argument Validation in SourceWorkflow for Default Source Type" (microsoft#4891). This commit incorporates part of the included fix for consistency.
- Updated the method of obtaining the default source type in the
winrt::Microsoft::Management::Deployment::implementation namespace
within PackageManager.cpp. Replaced the use of
::AppInstaller::Repository::ISourceFactory::GetForType("")->TypeName()
with ::AppInstaller::Repository::Source::GetDefaultSourceType().

This
change simplifies the process and ensures future-proofing against any
changes in the SourceFactory's default type.

src/Microsoft.Management.Deployment/PackageManager.cpp

@@ -984,8 +984,8 @@ namespace winrt::Microsoft::Management::Deployment::implementation
         if (sourceType.empty())
         {
             // This method of obtaining the default source type is slightly expensive as it requires creating a SourceFactory object
-            // and fetching the type name.Nonetheless, it future - proofs the code against any changes in the SourceFactory's default type.
-            sourceType = ::AppInstaller::Repository::ISourceFactory::GetForType("")->TypeName();
+            // and fetching the type name.Nonetheless, it future-proofs the code against any changes in the SourceFactory's default type.
+            sourceType = ::AppInstaller::Repository::Source::GetDefaultSourceType();
         }
 
         for (const auto& source : sourceList)