This brings the entire codebase and history of `gradle/gradle-build-action` into
the `gradle/actions` repository, after some modifications to make it easier to
merge.
This will permit the new `gradle/actions/setup-gradle` coordinates to carry on
where `gradle/gradle-build-action` leaves off.
- All NPM sources have been moved into a 'sources' directory
- The main action.yml and README are not located at `setup-gradle`