Canonicalize default-config-dir and plugin-path (#11999)

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# Description
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This PR makes sure `$nu.default-config-dir` and `$nu.plugin-path` are
canonicalized.

# User-Facing Changes
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helps us keep track of breaking changes. -->

`$nu.default-config-dir` (and `$nu.plugin-path`) will now give canonical
paths, with symlinks and whatnot resolved.

# Tests + Formatting
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I've added a couple of tests to check that even if the config folder
and/or any of the config files within are symlinks, the `$nu.*`
variables are properly canonicalized. These tests unfortunately only run
on Linux and MacOS, because I couldn't figure out how to change the
config directory on Windows. Also, given that they involve creating
files, I'm not sure if they're excessive, so I could remove one or two
of them.

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crates/nu-cli/src/config_files.rs

@@ -61,17 +61,18 @@ pub fn add_plugin_file(
     plugin_file: Option<Spanned<String>>,
     storage_path: &str,
 ) {
-    if let Some(plugin_file) = plugin_file {
-        let working_set = StateWorkingSet::new(engine_state);
-        let cwd = working_set.get_cwd();
+    let working_set = StateWorkingSet::new(engine_state);
+    let cwd = working_set.get_cwd();
 
+    if let Some(plugin_file) = plugin_file {
         if let Ok(path) = canonicalize_with(&plugin_file.item, cwd) {
             engine_state.plugin_signatures = Some(path)
         } else {
             let e = ParseError::FileNotFound(plugin_file.item, plugin_file.span);
             report_error(&working_set, &e);
         }
-    } else if let Some(mut plugin_path) = nu_path::config_dir() {
+    } else if let Some(plugin_path) = nu_path::config_dir() {
+        let mut plugin_path = canonicalize_with(&plugin_path, cwd).unwrap_or(plugin_path);
         // Path to store plugins signatures
         plugin_path.push(storage_path);
         plugin_path.push(PLUGIN_FILE);

crates/nu-protocol/src/eval_const.rs

@@ -29,7 +29,7 @@ pub fn create_nu_constant(engine_state: &EngineState, span: Span) -> Result<Valu
     let config_path = match nu_path::config_dir() {
         Some(mut path) => {
             path.push("nushell");
-            Ok(path)
+            Ok(canonicalize_path(engine_state, &path))
         }
         None => Err(Value::error(
             ShellError::ConfigDirNotFound { span: Some(span) },
@@ -121,7 +121,8 @@ pub fn create_nu_constant(engine_state: &EngineState, span: Span) -> Result<Valu
                     |e| e,
                     |mut path| {
                         path.push("plugin.nu");
-                        Value::string(path.to_string_lossy(), span)
+                        let canonical_plugin_path = canonicalize_path(engine_state, &path);
+                        Value::string(canonical_plugin_path.to_string_lossy(), span)
                     },
                 )
             },