Switch from HscSource to IsBootInterface for module lookup in GhcMake

We look up modules by their name, and not their contents. There is no way to separately reference a signature vs regular module; you get what you get. Only boot files can be referenced indepenently with import {-# SOURCE #-}.

• are either individually buildable or squashed
• have commit messages which describe what they do (referring to Notes and tickets using #NNNN syntax when appropriate)
• have added source comments describing your change. For larger changes you likely should add a Note and cross-reference it from the relevant places.
• add a testcase to the testsuite.
• replace this message with a description motivating your change

If you have any questions don't hesitate to open your merge request and inquire in a comment. If your patch isn't quite done yet please do add prefix your MR title with WIP:.