Zcu: remove *_loaded fields on File

Instead, `source`, `tree`, and `zir` should all be optional. This is
precisely what we're actually trying to model here; and `File` isn't
optimized for memory consumption or serializability anyway, so it's fine
to use a couple of extra bytes on actual optionals here.

src/Package/Module.zig

@@ -482,13 +482,11 @@ pub fn create(arena: Allocator, options: CreateOptions) !*Package.Module {
         };
         new_file.* = .{
             .sub_file_path = "builtin.zig",
-            .source = generated_builtin_source,
-            .source_loaded = true,
-            .tree_loaded = false,
-            .zir_loaded = false,
             .stat = undefined,
-            .tree = undefined,
-            .zir = undefined,
+            .source = generated_builtin_source,
+            .tree = null,
+            .zir = null,
+            .zoir = null,
             .status = .never_loaded,
             .prev_status = .never_loaded,
             .mod = new,