assistant: Remove some re-exports (#23262)

This PR removes some re-exports from the `assistant` to make it clearer
what its constituent modules depend on.

Release Notes:

- N/A

crates/assistant/src/slash_command.rs

@@ -1,8 +1,8 @@
 use crate::assistant_panel::ContextEditor;
-use crate::SlashCommandWorkingSet;
 use anyhow::Result;
 use assistant_slash_command::AfterCompletion;
 pub use assistant_slash_command::SlashCommand;
+use assistant_slash_command::SlashCommandWorkingSet;
 use editor::{CompletionProvider, Editor};
 use fuzzy::{match_strings, StringMatchCandidate};
 use gpui::{Model, Task, ViewContext, WeakView, WindowContext};