Cleaned up class loader

Eliminates _ci_loaded_files in favor of PHP require_once().

Only checks alternate cases of file name where necessary.

Checks both cases (if needed) of extension before iterating package paths.

Checks both cases (if needed) in each package path before proceeding to next path.

Checks that class exists and object doesn't before loading any config for class instantiation.

Signed-off-by: dchill42 <dchill42@gmail.com>