feat: add metapackage build support and fail-fast custom scripts

- add BuildType::Meta handling in builder and interactive spec output
- allow metapackage specs without sources and expose is_metapackage()
- avoid build deps in install planning for metapackages
- run custom non-function build.sh scripts with `sh -e` to stop on first failure
- add regression test for custom script fail-fast behavior
- apply small cleanup refactors (let-chains/iterator simplifications)

src/db/mod.rs

@@ -83,13 +83,13 @@ pub fn register_package(db_path: &Path, spec: &PackageSpec, destdir: &Path) -> R
             params![file],
             |row| row.get(0),
         );
-        if let Ok(owner) = owner_res {
-            if owner != spec.package.name {
-                if is_auto_removable_path(file) {
-                    auto_conflicts.push((file.clone(), owner));
-                } else {
-                    fatal_conflicts.push((file.clone(), owner));
-                }
+        if let Ok(owner) = owner_res
+            && owner != spec.package.name
+        {
+            if is_auto_removable_path(file) {
+                auto_conflicts.push((file.clone(), owner));
+            } else {
+                fatal_conflicts.push((file.clone(), owner));
             }
         }
     }