chore(pacmak): use Map instead of object (#3757)

---

By submitting this pull request, I confirm that my contribution is made under the terms of the [Apache 2.0 license].

[Apache 2.0 license]: https://www.apache.org/licenses/LICENSE-2.0

packages/jsii-pacmak/lib/targets/go/package.ts

@@ -379,7 +379,7 @@ export class RootPackage extends Package {
   public readonly assembly: Assembly;
   public readonly version: string;
   private readonly versionFile: VersionFile;
-  private typeCache: Record<string, GoType | undefined> = {};
+  private readonly typeCache = new Map<string, GoType | undefined>();
 
   // This cache of root packages is shared across all root packages derived created by this one (via dependencies).
   private readonly rootPackageCache: Map<string, RootPackage>;
@@ -460,18 +460,21 @@ export class RootPackage extends Package {
    * This allows resolving type references from other JSII modules
    */
   public findType(fqn: string): GoType | undefined {
-    if (!this.typeCache[fqn]) {
-      this.typeCache[fqn] = this.packageDependencies.reduce(
-        (accum: GoType | undefined, current: RootPackage) => {
-          if (accum) {
-            return accum;
-          }
-          return current.findType(fqn);
-        },
-        super.findType(fqn),
+    if (!this.typeCache.has(fqn)) {
+      this.typeCache.set(
+        fqn,
+        this.packageDependencies.reduce(
+          (accum: GoType | undefined, current: RootPackage) => {
+            if (accum) {
+              return accum;
+            }
+            return current.findType(fqn);
+          },
+          super.findType(fqn),
+        ),
       );
     }
-    return this.typeCache[fqn];
+    return this.typeCache.get(fqn);
   }
 
   /*