refactor(core): additional api cleanups

[eladb]

Additional minor API cleanups in the core module (see breaking changes list).

Updated physical names linter rule to verify that physical name property name is
cfnTypeName and that it's type is either PhysicalName in case the name is
optional and string if the name is required (fixes #2971).

Moved all internal files in @aws-cdk/cdk under "private"

BREAKING CHANGE: The deprecated app.run() has been removed (use app.synth()).

• core: CfnElement.refAsString renamed to ref of string type. The IResolvable version have been removed.
• core: Resource.physicalName is now a string instead of PhysicalName. If a physical name should be generated during deployment (i.e. not supplied to L1), the string will synthesize to undefined.
• core: IStringValue renamed to IStringProducer
• core: Include renamed to CfnInclude
• core: Cfn prefix was added to the following types: CfnCreationPolicy, CfnResourceAutoScalingCreationPolicy, CfnResourceAutoScalingCreationPolicy, CfnDeletionPolicy, CfnUpdatePolicy, CfnAutoScalingRollingUpdate, CfnAutoScalingReplacingUpdate, CfnAutoScalingScheduledAction, CfnCodeDeployLambdaAliasUpdate, CfnTag CfnRuleAssertion, CfnDynamicReferenceProps
• core: deepMerge is no longer exported.
• core: CfnOutputProps.export was renamed to exportName.
• core: CfnOutput all properties are now private
• core: StringListCfnOutput has been removed

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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.

[skinny85]

LGTM. 2 doc comments.

[skinny85]

Any thoughts on improving these names, like we talked about?

• arn -> attributeArn
• name -> attributeName
• arnComponents -> staticArnComponents

?

[mergify]

Thanks so much for taking the time to contribute to the AWS CDK ❤️

We will shortly assign someone to review this pull request and help get it
merged. In the meantime, please take a minute to make sure you follow this
checklist:

• PR title type(scope): text
  • type: fix, feat, refactor go into CHANGELOG, chore is hidden
  • scope: name of module without aws- or cdk- prefix or postfix (e.g. s3 instead of aws-s3-deployment)
  • text: use all lower-case, do not end with a period, do not include issue refs
• PR Description
  • Rationale: describe rationale of change and approach taken
  • Issues: indicate issues fixed via: fixes #xxx or closes #xxx
  • Breaking?: last paragraph: BREAKING CHANGE: <describe what changed + link for details>
• Testing
  • Unit test added. Prefer to add a new test rather than modify existing tests
  • CLI or init templates change? Re-run/add CLI integration tests
• Documentation
  • README: update module README to describe new features
  • API docs: public APIs must be documented. Copy from official AWS docs when possible
  • Design: for significant features, follow design process