Add property to disable conventional setup of loom

[lukebemish]

When loom's remapped dependencies are exposed across subprojects -- such as if a configuration based on a classpath is exposed from one project or another -- they cannot be consumed by projects without loom, as the local repositories loom sets up are not configured. A property to disable the "convention" setup of loom (setting up configurations/tasks/whatever) while maintaining the "ecosystem" aspects (the things that are needed for a loom project to work sensibly in an environment with other loom projects -- as far as I am aware, just repository setup) would be amazing. This can currently be accomplished to a degree through the use of loom as a settings plugin -- but these settings are overridden as soon as a project sets up any repositories manually, and it cannot be used on a per-project basis. I imagine such a property as, basically, simply providing an early return in plugin application -- likely here to be safest.