Remove non-top-level packages from PreprocessPackageDependenciesDesig…

…nTime

Performance work on the Dependencies node in dotnet/project-system mean
that only top-level package data is needed from the design-time build.

This commit removes items and metadata related to transitive
dependencies and hierarchy.

- Remove metadata Dependencies, which used to list child IDs
- Remove metadata IsTopLevelDependency, which is now implicitly true
- Remove 'target' items
- Remove object cloning, now that instances are immutable