[PP] Detect changes to selectors.yml

[jtcohen6]

Describe the bug

If I change selectors.yml and no other files, I still see:

Partial parsing enabled: 0 files deleted, 0 files added, 0 files changed.
Partial parsing enabled, no changes found, skipping parsing

This doesn't cause any big functional problems, such as selecting the wrong resources, because (I believe) selectors are re-parsed/rendered at runtime whenever a --selector flag is passed. It does matter, however, for saving the dict representation of selectors in manifest.json, which happens at parse time. That's increasingly important in a world where we support default selectors (#3448). It's quite confusing to update a yaml selector definition, dbt run --selector my_selector, see that it works, then check the manifest and see an outdated definition.

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