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-protocol/src/plugin/cache_file/mod.rs

@@ -96,11 +96,6 @@ impl PluginCacheFile {
                 .sort_by(|item1, item2| item1.name.cmp(&item2.name));
         }
     }
-
-    /// Remove a plugin from the plugin cache file by name.
-    pub fn remove_plugin(&mut self, name: &str) {
-        self.plugins.retain_mut(|item| item.name != name)
-    }
 }
 
 /// A single plugin definition from a [`PluginCacheFile`].