* Fix and refactor cd for Filesystem Shell.
Reorder check conditions, don't check existence twice.
If building for unix check exec bit on folder.
* Import PermissionsExt only on unix target.
* It seems that this is the correct way?
This improves incremental build time when working on what was previously
the root package. For example, previously all plugins would be rebuilt
with a change to `src/commands/classified/external.rs`, but now only
`nu-cli` will have to be rebuilt (and anything that depends on it).