Fix index_files being set False on Package Control updates

This commit persists old user value of `index_files` settings on disk as
global variables/states are vanished during Package Control updates, causing
`index_files` not being reset afterwards.

It uses json serialization to be able to extend backup usage in future,
without loosing backward compatibility. As Package Control blocks all other
updates, if it is going to update itself, it is very unlikely other settings
stored by PackageDisabler need to be persisted for the purpose of surviving
Package Control update.

There may however be other use cases to do so, which are not within scope 
of this fix.

package_control/package_disabler.py

@@ -22,6 +22,7 @@
     save_list_setting
 )
 from .show_error import show_error
+from .sys_path import pc_cache_dir
 
 
 class PackageDisabler:
@@ -162,8 +163,6 @@ class PackageDisabler:
     }
     """
 
-    index_files = None
-
     lock = RLock()
     restore_id = 0
 
@@ -441,9 +440,15 @@ def backup_and_reset_settings(packages, backup):
         # packages can be disabled/installed and re-enabled without indexer restarting
         # for each one individually. Also we don't want to re-index while a syntax
         # package is being disabled for upgrade - just once after upgrade is finished.
-        if backup and PackageDisabler.index_files is None:
-            PackageDisabler.index_files = settings.get('index_files')
-            settings.set('index_files', False)
+        if backup:
+            try:
+                # note: uses persistent cookie to survive PC updates.
+                with open(os.path.join(pc_cache_dir(), 'backup.json'), 'x', encoding='utf-8') as fobj:
+                    json.dump({'index_files': settings.get('index_files')}, fobj)
+            except OSError:
+                pass
+            else:
+                settings.set('index_files', False)
 
         # Backup and reset global theme(s)
         for key, default_file in PackageDisabler.default_themes.items():
@@ -539,9 +544,21 @@ def restore_settings(restore_id):
 
             try:
                 # restore indexing settings
-                if PackageDisabler.index_files is not None:
-                    settings.set('index_files', PackageDisabler.index_files)
-                    save_settings = True
+                fname = os.path.join(pc_cache_dir(), 'backup.json')
+                try:
+                    with open(fname, 'r', encoding='utf-8') as fobj:
+                        data = json.load(fobj)
+                        index_files = data.get('index_files')
+                        if isinstance(index_files, bool):
+                            settings.set('index_files', index_files)
+                            save_settings = True
+                except FileNotFoundError:
+                    pass
+                finally:
+                    try:
+                        os.remove(fname)
+                    except OSError:
+                        pass
 
                 # restore global theme
                 all_missing_theme_packages = set()
@@ -624,8 +641,6 @@ def restore_settings(restore_id):
                 if save_settings:
                     sublime.save_settings(preferences_filename())
 
-                PackageDisabler.index_files = None
-
                 PackageDisabler.color_scheme_packages = {}
                 PackageDisabler.theme_packages = {}