In-app/cross-stack references

[rix0rrr]

It should be possible to transparently reference resources in one stack from another stack, as long as both stacks are defined in the same CDK app. That is, the current export()/import() mechanism should not be necessary (which can then subsequently be co-opted for cross-app references).

Initially we will restrict to references between stacks in the same account and region (so that we can implement using Fn::ImportValue). A natural extension later on will be to extend to cross-account cross-region references.

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Requirements:

• References to cross-stack attributes turn into Exports on the providing side, and Fn::ImportValue on the importing side.
• References should record a dependency between stacks, so that:
  • We can reject cycles
  • cdk deploy can deploy multiple stacks in the right order.
• Mutations on passed objects (for the purposes of grant() etc) are still possible.

Non-requirements (but future extensions, tickets to be created later):

• Cross-account or cross-region references
• Solving stack dependency cycles (by automatically splitting single logical stack into multiple physical stacks).

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Some related but distinct tickets:

#1095 - exporting across CDK apps
#49 - cross region/account references
#928 - L2s: return a concrete ARN instead of Token if possible