Auto Init Marker Cached Modules

Auto-init now re-runs after a source change when modules are already cached

terragrunt plan/apply could fail with Error: Required plugins are not installed after a source-version change in any unit with a module "" block. The .terragrunt-init-required marker written on source change was being ignored because modulesNeedInit short-circuited as soon as .terraform/modules/ existed.

The marker check now lives at the top of needsInitRunCfg and is honored regardless of cached .terraform/modules/ contents.

Thanks to @arnaud-dezandee for contributing this fix!