feat(migrate): make as an independent executable

[abhi7cr]

Description of changes

Make the migrations tool a separate executable. This will remove the migrations package from CLI binary.

Note: Code for command generation and cross-cutting concerns (error handling, printing, formatting) is taken from gen2 codebase.

Commands:

 npx @aws-amplify/migrate to-gen-2 --help
migrate to-gen-2

Migrates an Amplify Gen1 app to a Gen2 app

Commands:
  migrate to-gen-2 generate-code  Generates Amplify Gen2 code based on Gen1
                                  configuration

Options:
  --debug  Print debug logs to the console            [boolean] [default: false]
  --help   Show help                                                   [boolean]

Issue #, if available

Description of how you validated changes

Checklist

• [x] PR description included
• [x] yarn test passes
• [x] Tests are changed or added
• [ ] Relevant documentation is changed or added (and PR referenced)
• [ ] New AWS SDK calls or CloudFormation actions have been added to relevant test and service IAM policies
• [ ] Pull request labels are added

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