Refactor AtomDB.get_matched_links to support nested lists in target_handles

[angeloprobst]

Currently, AtomDB.get_matched_links expects a list[str] in the param target_handles.

But there are situations where the caller could have a list of lists (ie. ['a', 'b', ['c', 'd'], 'e', [['f', 'g', 'h'], ['i', 'j'], 'k'], 'l']) - this can happen when links point to other links.
Which is the case in this stack:

LocalQueryEngine.fetch(...)
    LocalQueryEngine._process_link(...)
		LocalQueryEngine._generate_target_handles(...)  # <-- this function builds a list with nested lists
			<backend>.get_matched_links(...)

There are no tests on das-query-engine and das-atom-db covering this scenario.