Get rid of successfully_installed attr

src/pip/_internal/commands/install.py

@@ -291,7 +291,7 @@ class InstallCommand(RequirementCommand):
                         # installed from the sdist/vcs whatever.
                         wb.build(session=session, autobuilding=True)
 
-                    requirement_set.install(
+                    installed = requirement_set.install(
                         install_options,
                         global_options,
                         root=options.root_path,
@@ -305,9 +305,7 @@ class InstallCommand(RequirementCommand):
                         prefix=options.prefix_path,
                         isolated=options.isolated_mode,
                     )
-                    reqs = sorted(
-                        requirement_set.successfully_installed,
-                        key=operator.attrgetter('name'))
+                    reqs = sorted(installed, key=operator.attrgetter('name'))
                     items = []
                     for req in reqs:
                         item = req.name

src/pip/_internal/req/req_set.py

@@ -28,7 +28,6 @@ class RequirementSet(object):
         self.requirement_aliases = {}
         self.unnamed_requirements = []
         self.successfully_downloaded = []
-        self.successfully_installed = []
         self.reqs_to_cleanup = []
         self.use_user_site = use_user_site
         self.target_dir = target_dir  # set from --target option
@@ -243,4 +242,4 @@ class RequirementSet(object):
                         requirement.uninstalled_pathset.commit()
                 requirement.remove_temporary_source()
 
-        self.successfully_installed = to_install
+        return to_install