Use watchfiles as a file watching backend

[RealOrangeOne]

This doesn't use polling unless absolutely necessarily, making it more efficient. It also reduces the amount of first-party code required, and simplifies working out which files are being watched.

Pull Request Checklist

• Ensured tests pass and (if applicable) updated functional test output
• Conformed to code style guidelines by running appropriate linting tools
• Added tests for changed code
• Updated documentation for changed code

[avaris]

I'll review when I get the time but before that you also need to add it to the requirements list in setup.py.

[avaris]

Also, linter wants some changes.

[avaris]

Had to make some changes.

• For not-existing paths it was erroring out, so had to take them out of watch list
• Changed the order of pelican.run(), otherwise it was waiting awkwardly for a change before it run at the start.

[justinmayer]

Many thanks to @RealOrangeOne and @avaris for this enhancement! ✨