Support dvcignore for external dependencies.

[benjamintanweihao]

Current .dvcignore doesn't seem to apply to external dependencies. Our current use case is that we have models and data as external dependencies stored on an NFS drive (the cache is also on NFS), and there are certain datasets in /mnt/nfs/data that we do not want to expose to DVC.

[jorgeorpinel]

External outputs are also not supported by .dvcignore .

[efiop]

Closing as stale.