Resolves rdar://140298287
ExtractAPI's support for printing Objective-C category extensions from
other modules emits symbol graphs with an
`ExtendedModule@HostModule.symbols.json`. However, this is backwards
from existing symbol graph practices, causing issues when these symbol
graphs are consumed alongside symbol graphs generated with other tools
like Swift. This PR flips the naming scheme to be in line with existing
symbol graph tooling.
1be4a67454