Refactor UsdUtils_UsdzPackage into common base class.

This new class, UsdUtils_AssetLocalizationPackage will act as a base class for shared packaging functionality used by CreateNewUsdzPackage and to new forthcoming UsdUtilsLocalizaeAsset method.

This base class handles the following tasks in a generic way:
- Pprocessing dependencies and determining a relative package path
- Performing remapping in the case that a non-reative path is encountered during localization.
- Handling layers that were modified as part of the localization process
- Invoke a virtual function to add a resolved AR path to a package relative path

(Internal change: 2301707)

pxr/usd/usdUtils/CMakeLists.txt

@@ -35,6 +35,7 @@ pxr_library(usdUtils
     PRIVATE_CLASSES
         assetLocalization
         assetLocalizationDelegate
+        assetLocalizationPackage
 
     PUBLIC_HEADERS
         api.h