Skip files if no rules are enabled

[datur]

Summary

I had suggested on issue #365 that it would be nice to automatically skip a file if no rules were enabled for a given file #365 (comment).

To avoid unnecessary processing of files if no rules are enabled for a given file the config is now checked before processing a file and if no rules are enabled the file is skipped.

Test Plan

Two smoke tests have been added to confirm files are skipped if no rules are enabled.

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[amyreese]

There are two main issues with this approach:

1. by skipping the call to fixit_bytes(), we are not generating a clean Result() object for the file, which will result in the CLI output under-counting the number of clean file results.

2. this only catching cases where the file config has no enabled rules/modules, but doesn't prevent visiting files when the number of rules actually discovered at runtime is empty (eg, passing --tags to CLI or combination of enable/disable that results in no selected rules).

I think the better solution is to modify fixit_bytes() instead such that it generates a clean result and aborts early if the result of collect_rules(...) is empty.

[datur]

Thanks for the review @amyreese I believe I have addressed your change requests now! Let me know if you have any other feedback 😄

[amyreese]

Thank you!