Avoid repeated Element dependency graph traversal

[kotborealis]

Element.__update_cache_keys() may traverse the same BUILD dependency graph multiple times while calculating weak and strict cache keys.
PR #2172 reduces this locally by materializing the dependency list once and reusing it for both calculations, but Element._dependencies() could be cached with something like @functools.cache.

[nathanwilliams-ct]

From comment thread:
"""
hmm.. Instead of storing the result of _dependencies here, it might be worth refactoring _dependencies to directly cache it's own results, so it can return the cached result after the first call, whenever it's called anywhere instead of just here. I don't think __build_dependencies or __runtime_dependencies is ever updated after an Element is first initialised in _new_from_load_element. (Although _add_build_dependency might be a problem? that would need investigating, but you could clear the cache, in that method..)

...
  __dependency_set_cache: dict[str,set[Element]] = {}

def __dependencies(...) -> ...:

  # Check if we already calculated this set of dependencies
  dependency_set_cache_key = f"{scope}, {recurse}" # (probably need to include the _dependencies 'visited' argument here too...)
  if (dependencies := self.__dependency_set_cache.get(dependency_set_cache_key)) is not None:
     for element in dependencies:
       yield element
     return
  
   # Calculate result/visited
   .... 


   # Store result for next time
   self.__build_dependencies_set[dependency_set_cache_key] = result # ( or 'visited' from the recursive visit function)

It will need some thinking, but maybe it would even be appropriate to use functools @cache annotation? https://docs.python.org/3/library/functools.html The recursive visit function might also benefit from the @cache annotation..
"""