Implement package replacement feature across the codebase

- Introduced a new `replaces` field in various structs to support package replacements.
- Updated database schema to include a `replaces` table and corresponding queries.
- Modified the package index to handle replacements and ensure they take precedence over exact package names.
- Enhanced the `create_source_repo_index` function to include replacement entries in the index.
- Updated tests to validate the new replacement functionality, including parsing and metadata generation.
- Refactored relevant functions and structures to accommodate the new `replaces` feature in package specifications and alternatives.

src/package/packager.rs

@@ -192,6 +192,17 @@ impl Packager {
                     .collect(),
             ),
         );
+        map.insert(
+            "replaces".to_string(),
+            toml::Value::Array(
+                self.spec
+                    .alternatives
+                    .replaces
+                    .iter()
+                    .map(|s| toml::Value::String(s.clone()))
+                    .collect(),
+            ),
+        );
 
         // Add install-relevant dependency kinds for repo/runtime consumers.
         let mut deps = toml::map::Map::new();
@@ -434,6 +445,12 @@ mod tests {
         assert_eq!(val.get("version").and_then(|v| v.as_str()), Some("1.0"));
         assert_eq!(val.get("revision").and_then(|v| v.as_integer()), Some(1));
         assert_eq!(val.get("license").and_then(|v| v.as_str()), Some("MIT"));
+        assert!(
+            val.get("replaces")
+                .and_then(|v| v.as_array())
+                .expect("replaces should be an array")
+                .is_empty()
+        );
         assert!(
             crate::metadata_time::parse_completed_at_value(&val).is_some(),
             "expected RFC3339 UTC completed_at"
@@ -508,6 +525,27 @@ mod tests {
         assert_eq!(keep[1].as_str(), Some("etc/passwd"));
     }
 
+    #[test]
+    fn test_generate_metadata_toml_includes_replaces() {
+        let tmp = tempfile::tempdir().unwrap();
+        let dest = tmp.path();
+
+        let mut packager = mk_packager(dest.to_path_buf());
+        packager.spec.alternatives.replaces = vec!["findutils".into(), "diffutils".into()];
+        packager.generate_metadata_toml().unwrap();
+
+        let meta_path = dest.join(".metadata.toml");
+        let content = fs::read_to_string(meta_path).unwrap();
+        let val: toml::Value = toml::from_str(&content).unwrap();
+        let replaces = val
+            .get("replaces")
+            .and_then(|v| v.as_array())
+            .expect("replaces should be an array");
+        assert_eq!(replaces.len(), 2);
+        assert_eq!(replaces[0].as_str(), Some("findutils"));
+        assert_eq!(replaces[1].as_str(), Some("diffutils"));
+    }
+
     #[test]
     fn test_create_package_skips_purged_payload_paths() {
         let tmp = tempfile::tempdir().unwrap();