Cleanup: rename package manager addon from bpkg -> package_manager

As I see it, the *package manager* is called "bpkg", while the *package
manager addon* is called "package_manager".

The *package manager* contains the actual package management code, and
the *package manager addon* provides the interface to use it.

package_manager/cache.py

@@ -0,0 +1,49 @@
+# ##### BEGIN GPL LICENSE BLOCK #####
+#
+#  This program is free software; you can redistribute it and/or
+#  modify it under the terms of the GNU General Public License
+#  as published by the Free Software Foundation; either version 2
+#  of the License, or (at your option) any later version.
+#
+#  This program is distributed in the hope that it will be useful,
+#  but WITHOUT ANY WARRANTY; without even the implied warranty of
+#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#  GNU General Public License for more details.
+#
+#  You should have received a copy of the GNU General Public License
+#  along with this program; if not, write to the Free Software Foundation,
+#  Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+#
+# ##### END GPL LICENSE BLOCK #####
+
+
+import os
+import logging
+import pathlib
+
+from . import appdirs
+
+log = logging.getLogger(__name__)
+
+
+def cache_directory(*subdirs) -> pathlib.Path:
+    """Returns an OS-specifc cache location, and ensures it exists.
+
+    Should be replaced with a call to bpy.utils.user_resource('CACHE', ...)
+    once https://developer.blender.org/T47684 is finished.
+
+    :param subdirs: extra subdirectories inside the cache directory.
+
+    >>> cache_directory()
+    '.../blender_cloud/your_username'
+    >>> cache_directory('sub1', 'sub2')
+    '.../blender_cloud/your_username/sub1/sub2'
+    """
+
+    # TODO: use bpy.utils.user_resource('CACHE', ...)
+    # once https://developer.blender.org/T47684 is finished.
+    user_cache_dir = appdirs.user_cache_dir(appname='Blender', appauthor=False)
+    cache_dir = pathlib.Path(user_cache_dir) / 'blender_package_manager' / pathlib.Path(*subdirs)
+    cache_dir.mkdir(mode=0o700, parents=True, exist_ok=True)
+
+    return cache_dir