Fix keyword decorator for top level functions and decorator parsing

[jamofer]

This PR fixes the following issues related with keyword decorators:

• Search for keyword decorator in top level functions.
• Search for keyword decorator with relative/absolute path usage.
• Remove single quotes on keyword definition with decorator.

import robot.api.deco
from robot.api import deco

@robot.api.deco.keyword('This works')
def this_works():
    pass

@deco.keyword(name="And this")
def and_this():
    pass