Remove dependency `autorest`

Remove any dependencies, direct or indirect, on `autorest` and related packages. This tool is currently implemented as an autorest extension, so it would likely involve keeping as much of possible of the "core" logic and rules for validation, but replacing the autorest "driver" with custom code (perhaps using spec-model).

I'm hopeful this isn't too much work, because I think autorest is mostly used just to get the list of swagger files to analyze.