avoid importing a module twice

due to the way utils.import_module is implemented,
"ryu-manager ryu/app/switches.py" ends up with loading
switches.py module twice.
this commit fixes that by checking if module pathnames specified
on the command line is aliases of already loaded modules.
while the check is incomplete, it should cover the most of useful cases.

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>