Support import from certain artifact/image stage

Added ability to specify stage directive in the import directive to perform import from certain artifact or image stage:

import:
- add: /file
  to: /file
  artifact: myartifact
  before: setup
  stage: install

Note that image which imports some files from another image stage will be rebuild only if this specified stage has been rebuilt.

This feature allows to avoid unnecessary imports of unchanged files.

Implementation details

image stages-signature has been renamed to image content-signature;
added content-signature to each stage, which differs from regular signature and represents content of the stage.

Installation

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