Fixed #10006 -- Avoid inadvertently using third-party "json" module.

In r9707 we added a preference to use Python's standard json module (Python 2.6
and beyond). However, there is also a third-party module called json that was
being picked up by accident if it was installed. It isn't API compatible, so
bad things happened. This change avoids that problem.

Patch from markmuetz.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9910 bcc190cf-cafb-0310-a4f2-bffc1f526a37