feat(downloads): reduce scope of download agent

due to a miscommunication, the scope of the download agent has grown too
much. this commit reduces that scopes, and intends for a lot of the
heavy lifting to be done by the soon-to-be-implemented download manager.

src-tauri/src/downloads/mod.rs

@@ -1,5 +1,4 @@
 pub mod download_agent;
 pub mod download_commands;
 mod download_logic;
-mod manifest;
-pub mod progress;
+mod manifest;