Eagerly check listing/mapping in iterables (#752)

There is currently a bug around resolving variables within the iterable of a for
generator or spread syntax (https://github.com/apple/pkl/issues/741)

Normally, mappings/listings are type-checked lazily. However, this results in the said
bug getting widened, for any object members declared in the iterable.

As a workaround for now, prevent the bug from being any worse by ensuring that these
object members are eagerly typechecked.

pkl-core/src/test/files/LanguageSnippetTests/input/generators/forGeneratorNestedReference.pkl

@@ -0,0 +1,33 @@
+class Aviary {
+  birds: Listing<Bird>
+}
+
+class Bird {
+  age: Int
+}
+
+function Bird(_age: Int) = new Bird { age = _age }
+
+res1 {
+  for (i in IntSeq(1, 1)) {
+    for (j in 
+      new Aviary {
+        birds {
+          Bird(i)
+        }
+      }.birds
+    ) {
+      j
+    }
+  }
+}
+
+res2 {
+  for (i in IntSeq(1, 1)) {
+    ...new Aviary {
+      birds {
+        Bird(i)
+      }
+    }.birds
+  }
+}

pkl-core/src/test/files/LanguageSnippetTests/output/generators/forGeneratorNestedReference.pcf

@@ -0,0 +1,10 @@
+res1 {
+  new {
+    age = 1
+  }
+}
+res2 {
+  new {
+    age = 1
+  }
+}