Only define gethookproxy, isinitpath on Session

This fixes an issue where pylint complains about missing implementations
of abstract methods in subclasses of `File` which only override
`collect()` (as they should).

It is also cleaner and makes sense, these methods really don't need to
be overridden.

The previous methods defined directly on `FSCollector` and `Package` are
deprecated, to be removed in pytest 7.

See commits e2934c3f8c and
f10ab021e2 for reference.

src/_pytest/deprecated.py

@@ -84,3 +84,8 @@ WARNING_CAPTURED_HOOK = PytestDeprecationWarning(
     "The pytest_warning_captured is deprecated and will be removed in a future release.\n"
     "Please use pytest_warning_recorded instead."
 )
+
+FSCOLLECTOR_GETHOOKPROXY_ISINITPATH = PytestDeprecationWarning(
+    "The gethookproxy() and isinitpath() methods of FSCollector and Package are deprecated; "
+    "use self.session.gethookproxy() and self.session.isinitpath() instead. "
+)