* Custom canonicalize method for FilesystemShell.
* Use custom canonicalize method.
Fixed missing import.
* Move function body to already impl body.
* Create test that aims to resolve.
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).
