Use type's default for default stub map key

[knovichikhin]

Some models use specific non-primitive type
for a map key, such as an enum. Not resolving
to that results in a compilation failure.

Issue #, if available:

The particular issue is in smoke-aws. When building AppConfig model default instance ends up with:

     static let __default: AppConfigModel.CreateExtensionRequest = {
         let defaultInstance = AppConfigModel.CreateExtensionRequest(
            actions: ["Entry_0": [Action.__default]],
             description: nil,
             latestVersionNumber: nil,
             name: "0",

After this change the default instance will be:

     static let __default: AppConfigModel.CreateExtensionRequest = {
         let defaultInstance = AppConfigModel.CreateExtensionRequest(
            actions: [ActionPoint.__default: [Action.__default]],
             description: nil,
             latestVersionNumber: nil,
             name: "0",

Description of changes:

When building a default instance, resolve key to a specific default value for that type if possible.

Running smoke-aws-generate with this build results in the following changes:

--- a/Sources/AppConfigModel/AppConfigModelDefaultInstances.swift
+++ b/Sources/AppConfigModel/AppConfigModelDefaultInstances.swift
@@ -292,7 +292,7 @@ public extension CreateExtensionRequest {
      */
     static let __default: AppConfigModel.CreateExtensionRequest = {
         let defaultInstance = AppConfigModel.CreateExtensionRequest(
-            actions: ["Entry_0": [Action.__default]],
+            actions: [ActionPoint.__default: [Action.__default]],
             description: nil,
             latestVersionNumber: nil,
             name: "0",
diff --git a/Sources/CloudformationModel/CloudformationModelDefaultInstances.swift b/Sources/CloudformationM
odel/CloudformationModelDefaultInstances.swift
index 3deb8b9..c9baec4 100644
--- a/Sources/CloudformationModel/CloudformationModelDefaultInstances.swift
+++ b/Sources/CloudformationModel/CloudformationModelDefaultInstances.swift
@@ -2830,7 +2830,7 @@ public extension ResourceToImport {
     static let __default: CloudformationModel.ResourceToImport = {
         let defaultInstance = CloudformationModel.ResourceToImport(
             logicalResourceId: "value",
-            resourceIdentifier: ["Entry_0": "0"],
+            resourceIdentifier: ["0": "0"],
             resourceType: "0")

         return defaultInstance
diff --git a/Sources/DynamoDBModel/DynamoDBModelDefaultInstances.swift b/Sources/DynamoDBModel/DynamoDBModelDefaultInstances.swift
index 31cfced..9bf6dfc 100644
--- a/Sources/DynamoDBModel/DynamoDBModelDefaultInstances.swift
+++ b/Sources/DynamoDBModel/DynamoDBModelDefaultInstances.swift
@@ -280,7 +280,7 @@ public extension BatchGetItemInput {
      */
     static let __default: DynamoDBModel.BatchGetItemInput = {
         let defaultInstance = DynamoDBModel.BatchGetItemInput(
-            requestItems: ["Entry_0": KeysAndAttributes.__default],
+            requestItems: ["012": KeysAndAttributes.__default],
             returnConsumedCapacity: nil)

         return defaultInstance
@@ -350,7 +350,7 @@ public extension BatchWriteItemInput {
      */
     static let __default: DynamoDBModel.BatchWriteItemInput = {
         let defaultInstance = DynamoDBModel.BatchWriteItemInput(
-            requestItems: ["Entry_0": [WriteRequest.__default]],
+            requestItems: ["012": [WriteRequest.__default]],
             returnConsumedCapacity: nil,
             returnItemCollectionMetrics: nil)

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

[tachyonics]

LGTM, thanks for the fix!