"replace" directive support in PklProject

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.

Start by tracing how the PklProject manifest is read and how PklProject.deps.json is generated. Define the replacement behavior for transitive dependencies, including the error for creating packages when replace directives are present; done means dependency output reflects swaps and package creation rejects them.

Written by the indexing model from the issue text.