Switched caching to CacheControl + pillar module usable without Blender

Making the blender_cloud.pillar and blender_cloud.cache modules usable
without Blender required some moving of the code, from __init__.py to
blender.py.

CacheControl requires the lockfile package, which increases the number
of bundled wheel files to 3. Those are now managed by
blender_cloud.wheels.load_wheels(). The wheels are only loaded if the
Python installation doesn't yet contain the required packages. This allows
development with virtualenv-installed packages, debugging in the IDE, etc.

blender_cloud/cache.py

@@ -1,24 +1,15 @@
 """Cache management."""
 
 import os
-import sys
 import logging
+import requests
+import cachecontrol
+from cachecontrol.caches import FileCache
 
 from . import appdirs
 
 log = logging.getLogger(__name__)
-
-
-# Add our shipped Requests-Cache wheel to the Python path
-if not any('requests_cache' in path for path in sys.path):
-    import glob
-
-    # TODO: gracefully handle errors when the wheel cannot be found.
-    my_dir = os.path.dirname(__file__)
-    wheel = glob.glob(os.path.join(my_dir, 'requests_cache*.whl'))[0]
-    sys.path.append(wheel)
-
-import requests_cache
+_session = None  # requests.Session object that's set up for caching by requests_session().
 
 
 def cache_directory() -> str:
@@ -30,37 +21,27 @@ def cache_directory() -> str:
 
     # TODO: just use bpy.utils.user_resource('CACHE', ...)
 
-    cache_dir = os.path.join(appdirs.user_cache_dir(appname='Blender', appauthor=False), 'blender-cloud')
+    cache_dir = os.path.join(appdirs.user_cache_dir(appname='Blender', appauthor=False), 'blender_cloud')
 
     os.makedirs(cache_dir, exist_ok=True)
 
     return cache_dir
 
 
-def requests_session() -> requests_cache.CachedSession:
+def requests_session() -> requests.Session:
     """Creates a Requests-Cache session object."""
 
+    global _session
+
+    if _session is not None:
+        return _session
+
     cache_dir = cache_directory()
-    cache_name = os.path.join(cache_dir, 'blender_cloud_cache')
+    cache_name = os.path.join(cache_dir, 'blender_cloud_http')
     log.info('Storing cache in %s' % cache_name)
 
-    req_sess = requests_cache.CachedSession(backend='sqlite',
-                                            cache_name=cache_name)
+    _session = cachecontrol.CacheControl(sess=requests.session(),
+                                         cache=FileCache(cache_name))
 
-    return req_sess
+    return _session
 
-
-def debug_show_responses():
-
-    req_sess = requests_session()
-
-    log.info('Cache type: %s', type(req_sess.cache))
-    log.info('Cached URLs:')
-    for key in req_sess.cache.keys_map:
-        value = req_sess.cache.keys_map[key]
-        log.info('  %s = %s' % (key, value))
-
-    log.info('Cached responses:')
-    for key in req_sess.cache.responses:
-        response, timekey = req_sess.cache.get_response_and_time(key)
-        log.info('  %s = %s' % (key, response.content))