Rename contrib/arcanist to source-control-tools

Summary:
contrib/arcanist is a bad name because it doesn't describe what these tools are doing.
Rename to contrib/source-control-tools since these tools will take care of patching, rebasing, etc.

Test Plan:

ninja check-source-control-tools
ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4626