Changed the upload function to use the case-insensitive safe get_files function.

The upload function currently uses the shp_files function in geonode/geoserver/uploader/utils.py to identify sidecar files when given the path of a shapefile.  The shp_files function returns false positives on case-insensitive file systems which causes the uploader to break when using the importer backend.  The get_files function does the same thing as shp_files but works on case-insensitive file systems and is much more heavily tested.  No other logic in geonode uses shp_files.