add local directory support

[jakecoffman]

This adds the ability to copy a local directory into the updater to be used as the repository to be updated. The file fetcher will prefer to use the locally cloned repository and won't hit GitHub to fetch the files.

Copying in the directory has certain workflow advantages:

• Currently to test an update, you have to make a commit and push it to GItHub so the CLI can get to the files.
• Testing a small change to the repo also requires a new commit and push.
• It can save time if you have to run more than once.

Previously we tried using -v to mount the directory but this didn't work very well:

• Mounting the directory as writable could change the contents of what you're testing since some of the ecosystems do a git reset between dependency updates.
• Mounting as read-only also won't work since file operations are required.
• The mounting syntax is not easy to type -v "$(pwd):/home/dependabot/dependabot-updater/repo:ro"

The flag I chose for now is --local, but if you have a better idea let me know. The terms "directory", "repo", "path" are overloaded already so it was difficult to find a new term.

[brrygrdn]

Thanks for this, it makes a ton of sense.

I'm 👍🏻 on --local as the flag as we don't really have any other context other than local-dir right now, running against a local-host ( e.g. GHES? ) seems way out of scope and I think it can be achieved via the job definition if we needed to in a pinch.