pyang_plugins: Refactor

This patch improves the result of pylint and pep8 checks.
For example, this patch fixes;
 - "multiple_modules" attribute of "GolangPlugin" defined in out-side of
   "__init__()".
 - "module" shadows built-in name.
 - missing whitespace around operator.

Signed-off-by: IWASE Yusuke <[email protected]>