loader: Use ast_cli_completion_add for 'module load' completion.

This addresses all performance issues with 'module load' completion.  In
addition to using ast_cli_completion_add we stop using libedit's
filename_completion_function, instead using ast_file_read_dir.  This
ensures all results are produced from a single call to opendir.

Change-Id: I8bf51ffaa7ef1606f3bd1b5bb13f1905d72c6134