Fix amending module with abstract class (#319)

This fixes an assertion error that gets thrown if:
1. A module declares a class as abstract
2. An amending module does not use that abstract class as a type

Underneath the hood, the modifiers of the class/typelias object member
is considered different from the modifiers on the VmClass/VmTypeAlias
values.

The object model skips forcing any explicitly members that are explicitly
declared hidden, abstract, or local.
However, it _should_ evaluate any abstract classes found in a module.
So, it's incorrect to apply the same modifiers on the class to the object member.

pkl-core/src/test/files/LanguageSnippetTests/input-helper/modules/Birds.pkl

@@ -0,0 +1,12 @@
+module Birds
+
+abstract class Bird {
+  name: String
+}
+
+class Pigeon extends Bird {
+  name = "Pigeon"
+  passenger: Boolean
+}
+
+pidgy: Pigeon?

pkl-core/src/test/files/LanguageSnippetTests/input/modules/amendModule6.pkl

@@ -0,0 +1 @@
+amends ".../input-helper/modules/Birds.pkl"

pkl-core/src/test/files/LanguageSnippetTests/output/modules/amendModule6.pcf

@@ -0,0 +1 @@
+pidgy = null