refactor broadcast for flexible indexes #6481

What is your issue?

From @benbovy in #6477

• extract common indexes and explicitly pass them to the Dataset and DataArray constructors (when implemented) that are called in the broadcast helper functions (there are some temporary and ugly hacks in create_default_index_implicit so that it works now with pandas multi-indexes wrapped in coordinate variables without the need to pass those indexes explicitly)
• extract common indexes based on the dimension(s) of their coordinates and not their name (e.g., case of non-dimension but indexed coordinate)