skip hidden directories (start with .) picked up by -f

[danielhelfand]

Following up on https://github.com/vmware-tanzu/carvel-ytt/issues/240. The idea of excluding hidden directories with kbld -f I think is applicable as well as described in this comment.

[danielhelfand]

To follow up on why I think this is important for kbld, the use case is that kbld -f is used on an entire directory that also contains an .imgpkg directory.

If all resources are defined in a single file, kbld can get around not worrying about the contents of the .imgpkg directory by specifically targeting kbld -f at that single manifest and another -f at the imageslock file.

However, if resources are defined in several files (e.g. deployment.yml, service.yml) and I also want to pipe the result into kapp after running kbld -f, there should be a way to ignore the contents .imgpkg (such as a Bundle resource as described in the original ytt issue) while still being able to make use of the imageslock.