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

Cargo.lock

@@ -2622,6 +2622,7 @@ dependencies = [
  "anthropic",
  "anyhow",
  "assistant",
+ "assistant_slash_command",
  "assistant_tool",
  "async-stripe",
  "async-trait",
@@ -2678,6 +2679,7 @@ dependencies = [
  "pretty_assertions",
  "project",
  "prometheus",
+ "prompt_library",
  "prost 0.9.0",
  "rand 0.8.5",
  "recent_projects",
@@ -16246,6 +16248,7 @@ dependencies = [
  "project",
  "project_panel",
  "project_symbols",
+ "prompt_library",
  "proto",
  "recent_projects",
  "release_channel",