provider: fix vcs subdirectory propagation

This change ensures that when complete packages are created dependency
inherits subdirectory from package if supported.

poetry/puzzle/provider.py

@@ -187,6 +187,12 @@ class Provider:
         dependency._source_reference = package.source_reference
         dependency._source_reference = package.source_reference
         dependency._source_resolved_reference = package.source_resolved_reference
         dependency._source_resolved_reference = package.source_resolved_reference
 
 
+        if hasattr(package, "source_subdirectory") and hasattr(
+            dependency, "_source_subdirectory"
+        ):
+            # this is supported only for poetry-core >= 1.1.0a7
+            dependency._source_subdirectory = package.source_subdirectory
+
         self._deferred_cache[dependency] = package
         self._deferred_cache[dependency] = package
 
 
         return [package]
         return [package]