Refactor the PackageBuilder, separating the declared type support

The "declared types" support has grown and bloated over the years, from a minor feature of the PackageBuilder into a full sub-component.

• The declared types compiler should become an independent component, separate from the PackageBuilder
• it should be pluggable into the PackageBuilder, and extensible as needed
• it should be based on a core "declare" language, with appropriate extension points
• it should be (partially) rewritten, to remove inefficiencies, redundancies and inconsistencies due to years of patches and additions