Fix panic on missing first segment in package path resolution

When resolving a multi-segment package path like `foo:bar/nonexistent.{a}`,
if the first segment is not found in the package definitions, the code
would unwrap None and panic. Now breaks out of the lookup loop when the
first segment is not found, falling through to the existing error
reporting that produces a clear error message.

Added a resolution test case for this scenario.

Author: sumleo

crates/wac-parser/src/resolution.rs

@@ -2553,7 +2553,8 @@ impl<'a> AstResolver<'a> {
             }
 
             // Otherwise, project into the parent based on the current segment
-            let export = match found.unwrap() {
+            let Some(parent) = found else { break };
+            let export = match parent {
                 // The parent is an interface or instance
                 ItemKind::Type(Type::Interface(id)) | ItemKind::Instance(id) => {
                     state.graph.types()[id].exports.get(segment).copied()

crates/wac-parser/tests/resolution/fail/missing-package-definition.wac

@@ -0,0 +1,5 @@
+package test:comp;
+
+interface i {
+    use foo:bar/nonexistent.{a};
+}

crates/wac-parser/tests/resolution/fail/missing-package-definition.wac.result

@@ -0,0 +1,10 @@
+failed to resolve document
+
+  × package `foo:bar` has no export named `nonexistent`
+   ╭─[tests/resolution/fail/missing-package-definition.wac:4:17]
+ 3 │ interface i {
+ 4 │     use foo:bar/nonexistent.{a};
+   ·                 ─────┬─────
+   ·                      ╰── unknown export `nonexistent`
+ 5 │ }
+   ╰────

crates/wac-parser/tests/resolution/fail/missing-package-definition/foo/bar.wat

@@ -0,0 +1,3 @@
+(component
+  (import "foo" (func))
+)