Resolve dependency digests during build

[carolynvs]

During porter build, we should resolve the repository digest for each dependency that has a default bundle reference specified. The reference in the final porter.yaml stored in ./cnab/app/porter.yaml should use the digest instead of the tag.

Questions

[ ] It is still TBD how we will preserve the original tag in the bundle to use later for relocation mapping (#2670)

ℹ️ Read PEP003 - Advanced Dependencies for context about how dependencies should work, design details, and notes about desired behavior.