Warn when same functions get redefined in another spec

[tidoust]

The querySelector and querySelectorAll functions defined in Selectors API Level 1 are now part of DOM. If we have both specs in the crawl, the analysis should report that there is a problem here, the same way it reports on interfaces defined in multiple specs.

Note it may not be that easy to achieve, and may not be worth the hassle. In the example above, the methods are defined on parentNode in DOM, which does not exist in Selectors API Level 1. We'd have to follow the implements directives as well.