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/zed/Cargo.toml

@@ -87,6 +87,7 @@ profiling.workspace = true
 project.workspace = true
 project_panel.workspace = true
 project_symbols.workspace = true
+prompt_library.workspace = true
 proto.workspace = true
 recent_projects.workspace = true
 release_channel.workspace = true