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/planner.rs

@@ -601,8 +601,10 @@ impl<'a> Resolver<'a> {
 
 fn source_deps_for_install(spec: &PackageSpec) -> Vec<String> {
     let mut deps_all = Vec::new();
-    for dep in &spec.dependencies.build {
-        push_unique(&mut deps_all, dep.clone());
+    if !spec.is_metapackage() {
+        for dep in &spec.dependencies.build {
+            push_unique(&mut deps_all, dep.clone());
+        }
     }
     for dep in &spec.dependencies.runtime {
         push_unique(&mut deps_all, dep.clone());