1. Fixed a bug where `--all` wasn't showing files at the root directories.
2. More use of `Result`'s `map` and `map_err` methods.
3. Making tables be homogeneous so one can, for example, `get directories`.
Mostly, making more use of `map` and `map_err` in `Result`. One benefit
is that at least one location had duplicated logic for how to map the
error, which is no longer the case after this commit.
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).
