fix(amplify): addBranch fails synth with "cannot find entry file..." (#28658)

Error:

```bash
Error: Cannot find entry file at /node_modules/@aws-cdk/custom-resource-handlers/dist/
aws-amplify-alpha/asset-deployment-handler/index.js
```

In #27955, we migrated the aws-amplify-alpha custom resource to `@aws-cdk/custom-resource-handlers`, our internal package for storing all custom resources. However, this migration was half-baked. The reason is because we seem to have depended on our `@aws-cdk/custom-resource-handlers` package directly. That works locally, as we have `custom-resource-handlers` readily available. 

```
|-- @aws-cdk
    |-- aws-amplify-alpha
    |-- custom-resource-handlers
```

`aws-amplify-alpha` directly went into `custom-resource-handlers` to grab the necessary entry file. This works locally, because we have access to all folders with impunity.

Of course, when packaged and published, we publish only `aws-amplify-alpha` with no other outside folders:

```
|-- @aws-cdk 
    |-- aws-amplify-alpha
```

Thus, the published module would not have access to the file under `custom-resource-handlers`, and predictably fails at synth time. 

In `aws-cdk-lib`, we airlift the necessary files _into_ the package and release it alongside `aws-cdk-lib`:

```
|-- aws-cdk-lib
    |-- aws-synthetics
    |-- custom-resource-handlers/dist/aws-synthetics/custom-resource-handler // this was airlifted into aws-cdk-lib
```

We are supposed to do the same airlift mechanism for `aws-amplify-alpha` but somehow that was forgotten. This PR adds in the necessary structure, so now `aws-amplify-alpha` looks like this:

```
|-- @aws-cdk 
    |-- aws-amplify-alpha
        |-- custom-resource-handlers/dist/aws-amplify-alpha // airlifted in via this PR
    |-- custom-resource-handlers/dist
        |-- aws-amplify-alpha
```

(please excuse my horrible ascii notation)

Fixes #28633. Fixes #28089. I tested my locally packaged `aws-amplify-alpha` on a local CDK app to confirm that the necessary structure exists in the packaged module.


----

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

Author: kaizencc

packages/@aws-cdk/aws-amplify-alpha/.npmignore

@@ -31,3 +31,6 @@ junit.xml
 test/
 !*.lit.ts
 **/*.snapshot
+
+# include custom-resource-handlers
+!custom-resource-handlers/*

packages/@aws-cdk/aws-amplify-alpha/lib/branch.ts

@@ -233,7 +233,7 @@ class AmplifyAssetDeploymentProvider extends NestedStack {
       this,
       'amplify-asset-deployment-on-event',
       {
-        entry: path.join(__dirname, '..', '..', 'custom-resource-handlers', 'dist', 'aws-amplify-alpha', 'asset-deployment-handler', 'index.js'),
+        entry: path.join(__dirname, '..', 'custom-resource-handlers', 'dist', 'aws-amplify-alpha', 'asset-deployment-handler', 'index.js'),
         handler: 'onEvent',
         initialPolicy: [
           new iam.PolicyStatement({

packages/@aws-cdk/aws-amplify-alpha/package.json

@@ -67,7 +67,10 @@
   "cdk-build": {
     "env": {
       "AWSLINT_BASE_CONSTRUCT": true
-    }
+    },
+    "pre": [
+      "./scripts/airlift-custom-resource-handlers.sh"
+    ]
   },
   "keywords": [
     "aws",

packages/@aws-cdk/aws-amplify-alpha/scripts/airlift-custom-resource-handlers.sh

@@ -0,0 +1,23 @@
+#!/bin/bash
+
+scriptdir=$(cd $(dirname $0) && pwd)
+customresourcedir=$(node -p "path.dirname(require.resolve('@aws-cdk/custom-resource-handlers/package.json'))")
+awscdklibdir=${scriptdir}/..
+
+list_custom_resources() {
+  for file in $customresourcedir/dist/aws-amplify-alpha/*/index.js; do
+    echo $file | rev | cut -d "/" -f 2-4 | rev
+  done
+}
+
+customresources=$(list_custom_resources)
+
+echo $customresources
+
+cd $awscdklibdir
+mkdir -p $awscdklibdir/custom-resource-handlers
+
+for cr in $customresources; do
+  mkdir -p $awscdklibdir/custom-resource-handlers/$cr
+  cp $customresourcedir/$cr/index.js $awscdklibdir/custom-resource-handlers/$cr
+done

packages/@aws-cdk/aws-amplify-alpha/test/integ.app-asset-deployment.js.snapshot/asset.85f7bb0ac0b4da3571afd49c891d631f4f07c0ef710a02046d3a9abd8d9d4604/index.js

@@ -90,7 +90,7 @@
        {
         "Fn::Sub": "cdk-hnb659fds-assets-${AWS::AccountId}-${AWS::Region}"
        },
-       "/d1af98ce124bb93199d3d45ff8930b99badcdd089777b7976a0ad9af56d220f6.json"
+       "/2f9d2632b48a4f92f6691c3e3cfcbeb47798c945a2d03b93514de1a3d3ec72ed.json"
       ]
      ]
     }

packages/@aws-cdk/aws-amplify-alpha/test/integ.app-asset-deployment.js.snapshot/cdk-amplify-app-asset-deployment.assets.json

@@ -64,7 +64,7 @@
      "S3Bucket": {
       "Fn::Sub": "cdk-hnb659fds-assets-${AWS::AccountId}-${AWS::Region}"
      },
-     "S3Key": "9307ab790c544d7c4d94504a1c950222bbfc119c2c12b44440ad46939e0b6364.zip"
+     "S3Key": "85f7bb0ac0b4da3571afd49c891d631f4f07c0ef710a02046d3a9abd8d9d4604.zip"
     },
     "Environment": {
      "Variables": {

packages/@aws-cdk/aws-amplify-alpha/test/integ.app-asset-deployment.js.snapshot/cdk-amplify-app-asset-deployment.template.json

@@ -52,7 +52,6 @@
   "AppF1B96344": {
    "Type": "AWS::Amplify::App",
    "Properties": {
-    "Name": "App",
     "BasicAuthConfig": {
      "EnableBasicAuth": false
     },
@@ -62,13 +61,14 @@
       "Arn"
      ]
     },
+    "Name": "App",
+    "Platform": "WEB",
     "Repository": {
      "Fn::GetAtt": [
       "Repo02AC86CF",
       "CloneUrlHttp"
      ]
-    },
-    "Platform": "WEB"
+    }
    }
   },
   "AppmainF505BAED": {
@@ -120,4 +120,4 @@
    ]
   }
  }
-}
+}