Fixed bugs with package.d files and mixins.

The first issue I fixed is caused by use of a package.d file ([described here](http://dlang.org/changelog.html#import_package)). ddox would find the module which shares its name with a package, and crash.

The second error is caused when using using mixins. I'm not sure exactly what the issue here is, and this is not a permanent fix, but it does get the job done. It simply returns null when a mixin is found. This solves issue #37.