DynamoDB Local is not available for stage when loading from file

**Actual Behaviour**
When running:
```bash
serverless dynamodb migrate --stage test
```
We get:
```
Serverless: Skipping migration: DynamoDB Local is not available for stage: test
```

**Expected Behaviour**

It would work as it used to

**Steps to reproduce it**

- Create a new file for dynamodb-local config, let's call it `dynamodb-local.yml`
- Add it to serverless.yml as follows:
```yml
custom:
dynamodb: ${file(./dynamodb-local.yml)}
```

**LogCat for the issue**

```
Serverless: Skipping migration: DynamoDB Local is not available for stage: test
```

**Screenshots of the issue**

None

**Would you like to work on the issue?**

Yes

---
***temp solution***
the solution for me was reverting back to v0.2.35

Start by reproducing `serverless dynamodb migrate --stage test` with the external `dynamodb-local.yml` loaded from `serverless.yml`, and compare the behavior with version 0.2.35. Trace how the plugin determines whether DynamoDB Local is available for the requested stage; done means the migration runs for this configuration instead of being skipped.

Written by the indexing model from the issue text.