This is a small performance improvement in which an ajv validator for the
ng-package JSON schema is only compiled once. Additionally, the schema
validation logic is refactored into its own file.

This commit introduces a cache of which entry-points have already been
processed by ngcc that is used across entry-points, to avoid the overhead
of invoking ngcc repeatedly for all entry-points.

As the analysis file cache was previously never invalidated, any newly
introduced imports would not be detected in the analysis phase. This could
result in e.g. circular imports or self imports to go unnoticed during
watch builds.