Some log -> log_debug changes to quieten messages that don't tend to be useful.

So far, 

- `main_module` now only `->log`s when guessing,
- `main_module` calls `->log_debug` about the used module in all cases.
- `abstract` calls `->log_debug` instead of `->log` to indicate its getting `abstract` from somewhere else.

There's probably a few more I can clean up, but these ones are the most obvious and seemingly less contravercial ones.