…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*