rblib/modules: remove NotedPkg class and instead attach the notes to Build

The name ('Build') is a tad sad, but it's de-facto the object
representing the suite/arch instance of a package.
Also, before we would repeatidly load and overwrite the notes
information in the Package.note attribute, now it's actually doing
something sane

Signed-off-by: Mattia Rizzolo <mattia@debian.org>