When extending another module's type in your module, serialize declarations in
the extension into the other module's "extension" symbol graph file, including
relationships. This mechanic should continue up to the rootmost module. For
example:
A.AStruct <- B.BStruct < C.CStruct
Both BStruct and CStruct should go in `@A` symbol graph files because AStruct
owns BStruct and by extension owns CStruct. This is reflected in
documentation curation in some form already.
rdar://60796811
When a module extends a type from another module, serialize those symbols into
separated files dedicated to those extended modules. This makes it easier to
ingest and categorize those symbols under the extended module if desired.
rdar://58941718