mach: Avoid reopening the file for relocation

This commit allows the relocation code to perform install name
and dylib ID changes without reopening the file separately.

It's helpful to note that the undeclared/unnecessary dependencies are
done based on linkage alone.

linkage_checker: add a check for extraneous dependencies

- rename 'extraneous' to 'unnecessary'
- add the report under `linkage --test`