Implement update check functionality with version comparison and archive listing

- Added `check.rs` to handle package update checks, including logic for determining available updates from remote git repositories and archive listings.
- Introduced `versions.rs` to manage version patterns, comparison logic, and extraction of candidate versions from git refs and archive listings.
- Implemented `hex.rs` for encoding byte arrays to lowercase hexadecimal strings.

src/source/git.rs

@@ -202,7 +202,7 @@ fn mirror_key(url: &str) -> String {
     let mut hasher = Sha256::new();
     hasher.update(url.as_bytes());
     let digest = hasher.finalize();
-    format!("{:x}", digest)
+    crate::hex::encode_lower(digest)
 }
 
 fn ensure_mirror(