Sets don't allow duplicates, so checking for `has` is not necessary

webpack 5 makes these Sets lazy, which makes reading more expensive.
So only the watcher should read from these Sets, which is called after the compilation has finished,
so that doesn't contribute to build performance.

LazySets also have a `addAll` method, which allows to add an Iterable lazily (only iterated when LazySet is read)

use includes to check for existence