Use directories for autoloading (#13382)

fixes https://github.com/nushell/nushell/issues/13378

# Description

This PR tries to improve usage of system APIs to determine the location
of vendored autoload files.

# User-Facing Changes
The paths listed in #13180 and #13217 are changing. This has not been
part of a release yet, so arguably the user facing changes are only to
unreleased features anyway.

# Tests + Formatting
Haven't done, but if someone wants to help me here, I'm open to doing
it. I just don't know how to properly test this.

# After Submitting

crates/nu-cli/tests/completions/mod.rs

@@ -833,7 +833,7 @@ fn variables_completions() {
         "plugin-path".into(),
         "startup-time".into(),
         "temp-path".into(),
-        "vendor-autoload-dir".into(),
+        "vendor-autoload-dirs".into(),
     ];
 
     // Match results