Prefer stdlib importlib over Django's included version

The app_loading module had to shuffle things a bit. When it was
importing the function it raised a [RuntimeError][]. Simply importing
the module resolved that.

[RuntimeError]: https://gist.github.com/benspaulding/f36eaf483573f8e5f777

Author: benspaulding

haystack/templatetags/highlight.py

@@ -5,7 +5,10 @@
 from django import template
 from django.conf import settings
 from django.core.exceptions import ImproperlyConfigured
-from django.utils import importlib, six
+from django.utils import six
+
+from haystack.utils import importlib
+
 
 register = template.Library()
 

haystack/utils/__init__.py

@@ -11,9 +11,12 @@
 from haystack.utils.highlighting import Highlighter
 
 try:
-    from django.utils import importlib
-except ImportError:
+    # Introduced in Python 2.7
     import importlib
+except ImportError:
+    # Deprecated in Django 1.8; removed in Django 1.9 (both of which require
+    # at least Python 2.7)
+    from django.utils import importlib
 
 IDENTIFIER_REGEX = re.compile('^[\w\d_]+\.[\w\d_]+\.\d+$')
 

haystack/utils/app_loading.py

@@ -5,7 +5,9 @@
 from django.conf import settings
 from django.core.exceptions import ImproperlyConfigured
 from django.db.models.loading import get_app, get_model, get_models
-from django.utils.importlib import import_module
+
+from haystack.utils import importlib
+
 
 __all__ = ['haystack_get_models', 'haystack_load_apps']
 
@@ -56,7 +58,7 @@ def is_app_or_model(label):
 
     def haystack_get_app_modules():
         """Return the Python module for each installed app"""
-        return [import_module(i) for i in settings.INSTALLED_APPS]
+        return [importlib.import_module(i) for i in settings.INSTALLED_APPS]
 
     def haystack_load_apps():
         # Do all, in an INSTALLED_APPS sorted order.

haystack/utils/loading.py

@@ -8,11 +8,11 @@
 
 from django.conf import settings
 from django.core.exceptions import ImproperlyConfigured
-from django.utils import importlib
 from django.utils.datastructures import SortedDict
 from django.utils.module_loading import module_has_submodule
 
 from haystack.exceptions import NotHandled, SearchFieldError
+from haystack.utils import importlib
 from haystack.utils.app_loading import haystack_get_app_modules