GH action to respect working-directory

[akaihola]

🟠 Pending review is delaying Darker 3.0.0 release.

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This patch introduces the following changes to the Darker GitHub Action:

• new working-directory input option (defined in action.yml)
• new exitcode and stdout outputs, and Python helpers for setting them
• use the commit-range action straight from the current Darker Git checkout
• install pip-requirements-parser in a separate action step instead of the entrypoint script
• cache the Python environment
• exit with code 21 if specified working-directory doesn't exist
• refactor pip install into a helper function
• only create virtualenv if it doesn't already exist
• correct GitHub URL when version starts with @ (pointing to a branch or tag)
• run Darker in the specified working-directory
• return Darker terminal output in the stdout output
• check exit code and output in the working-directory test workflow
• update action tests to match new behavior

FIxes #587

Goes into Darker 3.0.0 since exit codes will need to change in a backwards incompatible way.

[akaihola]

@dshemetov this one is finally ready for review and validation! The working-directory: input option should now work in the GitHub action.

[akaihola]

I'm still hoping for extra eyeballs for this one. Sorry for adding quite a bit of mechanics necessary for testing this feature. Let me know if more comments or supporting documentation is required.

This may be easier to review commit by commit than the whole diff at once.

Ping @dshemetov @Svenito @dwt @MatthijsBurgh whichever one of you may be able to spare some time on reviewing.

[akaihola]

Now rebased on master after baseline linting removal.

[MatthijsBurgh]

Sorry, I don't have time for this one

[akaihola]

Still looking for a reviewer for this PR. You seem to have the ball rolling @clintonsteiner (thanks for all the effort!), would you be interested to look at this change to the GitHub action as well?