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

tests/plugin_persistence/mod.rs

@@ -72,6 +72,17 @@ fn plugin_process_exits_after_stop() {
     );
 }
 
+#[test]
+fn plugin_stop_can_find_by_filename() {
+    let result = nu_with_plugins!(
+        cwd: ".",
+        plugin: ("nu_plugin_inc"),
+        r#"plugin stop (plugin list | where name == inc).0.filename"#
+    );
+    assert!(result.status.success());
+    assert!(result.err.is_empty());
+}
+
 #[test]
 fn plugin_process_exits_when_nushell_exits() {
     let out = nu_with_plugins!(