re-useable StageDeclaration in `AWS::CodePipeline::Pipeline`

### Name of the resource

Other

### Resource name

AWS::CodePipeline::Pipeline

### Description

Currently, our build pipelines all look like:

* Bitbucket source
* self-update
* // actually distinct stuff //
* publish artifact to S3

Currently, the definitions for the three uniform steps have to be repeated in every CloudFormation template - giving us a lot of room for manual error, making them much busier to read, and complicating maintenance. Being able to deduplicate that definition would be extremely useful. One way this could be done is by making them a separate resource, and allowing them to either be defined on the `Pipeline` inline (maintaining backwards compatibility) or with a `Ref`.

### Other Details

https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-codepipeline-pipeline.html#cfn-codepipeline-pipeline-stages

https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-codepipeline-pipeline-stages.html

https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/modules.html

No repository files or tests are named. Start with the AWS::CodePipeline::Pipeline stages property, the linked StageDeclaration documentation, and the CloudFormation modules documentation. Done means the requested deduplication approach and its compatibility with inline definitions are clearly established.

Written by the indexing model from the issue text.