* Add support for recursively resolving directory symlinks.
This adds support for properly resolving directory symlinks within kodata.
I verified that with this I can symlink `.git/refs` into `kodata/` and (with
changes) resolve the `.git/HEAD` symlink to read the appropriate
`ref: refs/heads/...` files with the commit SHA.
* Incorporate code review feedback
When ko is invoked in this mode, import paths must have the `ko://`
prefix. If a human marks an import path with `ko://` and ko can't
resolve the resulting import path, it fails. In "loose mode", such an
import path would be silently ignored and passed on to the resolved
YAML, often resulting in invalid image names (e.g., `image:
github.com/foo/bar`)
In loose mode, `ko://` prefixes are always ignored for
backward-compatibility.
