Refactoring NuGet package specifications

The centralized specs folder is disjoint from just offering nuspecs and the associated version information alongside projects, inside their respective folders.

This makes diffs much more clear: only the affected component's folder is updated.

With the change to revisit how the refactored authoring works, I also updated the build system that "burns" the NuGet specifications: instead of simply burning a permanent change in a real build, separate specs are stored as build artifacts and used.