Migrate to monorepo

[mrook]

The current multi-repo setup is causing more trouble than it is solving. There is a significant maintenance burden to keep individual task packages up-to-date, release new versions, and debuga/test task packages.

I've therefor made the choice to revert back to a monorepo setup, where individual task packages are published as read-only subtree splits.

Pre-migration:

• Make all task repos read-only
• Prepare on-push script for subtree splits (https://github.com/symplify/monorepo-split-github-action)

Migration:

• Add monorepo-builder dep (https://github.com/symplify/monorepo-builder)
• Move task source files from Task/XYZ/src/ to Task/XYZ/
• Fix namespace of SVN tasks
• Fix composer.json
• Add all tasks/types back to taskdef.properties / typedef.properties? Or do something with custom properties?
• Enable on-push script
• Make all task- repositories read only (disable issues, wiki, pull requests)
• Update task- README files reflecting split/read-only status