Accept filenames in other plugin management commands (#12639)

# Description

This allows the following commands to all accept a filename instead of a
plugin name:

- `plugin use`
- `plugin rm`
- `plugin stop`

Slightly complicated because of the need to also check against
`NU_PLUGIN_DIRS`, but I also fixed some issues with that at the same
time

Requested by @fdncred

# User-Facing Changes

The new commands are updated as described.

# Tests + Formatting

Tests for `NU_PLUGIN_DIRS` handling also made more robust.

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting

- [ ] Double check new docs to make sure they describe this capability

crates/nu-path/src/lib.rs

@@ -4,7 +4,7 @@ mod helpers;
 mod tilde;
 mod util;
 
-pub use expansions::{canonicalize_with, expand_path_with, expand_to_real_path};
+pub use expansions::{canonicalize_with, expand_path_with, expand_to_real_path, locate_in_dirs};
 pub use helpers::{config_dir, config_dir_old, home_dir};
 pub use tilde::expand_tilde;
 pub use util::trim_trailing_slash;