"replace" directive support in PklProject #1302
Description
Goal: allow swapping transitive dependencies altogether (beyond just a version bump).
The PklProject manifest should have some replace directive that allows users to swap one package with another package.
This will then influence how PklProject.deps.json is generated.
Note: it should be an error to create packages when there are replace directives present, because packages shouldn't be allowed to control replacements of downstream projects.