fix(route53-targets): move Alias Targets into their own package

[rix0rrr]

In accordance with new guidelines, we're centralizing cross-service
integrations into their own package. In this case, centralizing
Route53 Alias Targets into @aws-cdk/aws-route53-targets.

Fixes #2448.

BREAKING CHANGE: using a CloudFront Distribution or an ELBv2
Load Balancer as an Alias Record Target now requires an integration
object from the @aws-cdk/aws-route53-targets package.

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Pull Request Checklist

• Testing
  • Unit test added (prefer not to modify an existing test, otherwise, it's probably a breaking change)
  • CLI change?: coordinate update of integration tests with team
  • cdk-init template change?: coordinated update of integration tests with team
• Docs
  • jsdocs: All public APIs documented
  • README: README and/or documentation topic updated
  • Design: For significant features, design document added to design folder
• Title and Description
  • Change type: title prefixed with fix, feat and module name in parens, which will appear in changelog
  • Title: use lower-case and doesn't end with a period
  • Breaking?: last paragraph: "BREAKING CHANGE: <describe what changed + link for details>"
  • Issues: Indicate issues fixed via: "Fixes #xxx" or "Closes #xxx"
• Sensitive Modules (requires 2 PR approvers)
  • IAM Policy Document (in @aws-cdk/aws-iam)
  • EC2 Security Groups and ACLs (in @aws-cdk/aws-ec2)
  • Grant APIs (only if not based on official documentation with a reference)

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

[RomainMuller]

Hardcore nitpicking/bikeshedding, but I'd really like it if we could keep this list sorted alphabetically. Feel free to ignore though (it's not so important in the grand scheme of things).

[eladb]

So much cleaner. I am happy with this!