prefer to use _features than _extras

anticipating that
<python-poetry/poetry-core#370> will remove
_extras

src/poetry/packages/locker.py

@@ -268,7 +268,7 @@ def __walk_dependencies(
             extras = requirement.extras
             requirement = locked_package.to_dependency()
             requirement.marker = requirement.marker.intersect(marker)
-            requirement._extras = extras
+            requirement._features = extras
 
             requirement.set_constraint(constraint)
 
@@ -289,7 +289,7 @@ def __walk_dependencies(
                 combined_requirement.marker = combined_requirement.marker.union(
                     requirement.marker
                 )
-                combined_requirement._extras |= requirement.extras
+                combined_requirement._features |= requirement._features
 
         return nested_dependencies
 
@@ -370,8 +370,8 @@ def get_project_dependency_packages(
             project_requires=selected,
             locked_packages=repository.packages,
         ):
-            if dependency.extras:
-                package = package.with_features(dependency.extras)
+            if dependency.features:
+                package = package.with_features(dependency.features)
 
             yield DependencyPackage(dependency=dependency, package=package)