main: inline Session._perform_collect() into perform_collect()

It doesn't add much, mostly just an eye sore, particularly with the
overloads.

src/_pytest/main.py

@@ -552,41 +552,20 @@ class Session(nodes.FSCollector):
         in which case the return value contains these collectors unexpanded,
         and ``session.items`` is empty.
         """
-        hook = self.config.hook
-        try:
-            items = self._perform_collect(args, genitems)
-            self.config.pluginmanager.check_pending()
-            hook.pytest_collection_modifyitems(
-                session=self, config=self.config, items=items
-            )
-        finally:
-            hook.pytest_collection_finish(session=self)
-        self.testscollected = len(items)
-        return items
-
-    @overload
-    def _perform_collect(
-        self, args: Optional[Sequence[str]], genitems: "Literal[True]"
-    ) -> List[nodes.Item]:
-        ...
-
-    @overload  # noqa: F811
-    def _perform_collect(  # noqa: F811
-        self, args: Optional[Sequence[str]], genitems: bool
-    ) -> Union[List[Union[nodes.Item]], List[Union[nodes.Item, nodes.Collector]]]:
-        ...
-
-    def _perform_collect(  # noqa: F811
-        self, args: Optional[Sequence[str]], genitems: bool
-    ) -> Union[List[Union[nodes.Item]], List[Union[nodes.Item, nodes.Collector]]]:
         if args is None:
             args = self.config.args
+
         self.trace("perform_collect", self, args)
         self.trace.root.indent += 1
+
         self._notfound = []  # type: List[Tuple[str, NoMatch]]
-        initialpaths = []  # type: List[py.path.local]
         self._initial_parts = []  # type: List[Tuple[py.path.local, List[str]]]
         self.items = items = []  # type: List[nodes.Item]
+
+        hook = self.config.hook
+
+        try:
+            initialpaths = []  # type: List[py.path.local]
             for arg in args:
                 fspath, parts = resolve_collection_argument(
                     self.config.invocation_dir, arg, as_pypath=self.config.option.pyargs
@@ -604,11 +583,22 @@ class Session(nodes.FSCollector):
                     errors.append("not found: {}\n{}".format(arg, line))
                 raise UsageError(*errors)
             if not genitems:
-            return rep.result
+                # Type ignored because genitems=False is only used by tests. We don't
+                # want to change the type of `session.items` for this case.
+                items = rep.result  # type: ignore[assignment]
             else:
                 if rep.passed:
                     for node in rep.result:
                         self.items.extend(self.genitems(node))
+
+            self.config.pluginmanager.check_pending()
+            hook.pytest_collection_modifyitems(
+                session=self, config=self.config, items=items
+            )
+        finally:
+            hook.pytest_collection_finish(session=self)
+
+        self.testscollected = len(items)
         return items
 
     def collect(self) -> Iterator[Union[nodes.Item, nodes.Collector]]: