Source-loader: Fix export default variable references

[shilman]

Issue: #20421

What I did

Fixed source-loader to handle the pattern:

const meta = { ... };
export default meta;

// ...stories

I did this because we're now using this pattern in our CLI template stories & as the recommended pattern in 7.0. Even though we're not actively maintaining/developing source-loader, this is a very basic fix.

How to test

• See attached unit test

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

[tmeasday]

LGTM