Formally deprecate the memorycache module

In change I97dcd55838941de4bf31aa84b4af7b2cb8e4f116 it was pointed out
that memorycache is deprecated and people should be moving onto using
the oslo.cache library.

This change uses debtcollector to log an actual deprecation warning for
those using the memorycache module at runtime.

Change-Id: Ib013c7b6586860944268b9eec41c6ae003f5e0c8

openstack/common/memorycache.py

@@ -18,6 +18,7 @@
 
 import copy
 
+from debtcollector import removals
 from oslo_config import cfg
 from oslo_utils import timeutils
 
@@ -30,6 +31,11 @@ CONF = cfg.CONF
 CONF.register_opts(memcache_opts)
 
 
+# Indicate that this module is deprecated for removal and oslo.cache should
+# be used instead.
+removals.removed_module(__name__, 'oslo.cache')
+
+
 def list_opts():
     """Entry point for oslo-config-generator."""
     return [(None, copy.deepcopy(memcache_opts))]

requirements.txt

@@ -4,6 +4,7 @@
 
 pbr<2.0,>=1.6
 argparse
+debtcollector>=0.3.0  # Apache-2.0
 greenlet>=0.3.2
 Jinja2>=2.6 # BSD License (3 clause)
 oslo.concurrency>=2.3.0 # Apache-2.0