Make referenced-builder contract checks visibility-aware #313

Sub-task of #301.

Goal

The contract checks in BuilderScopeResolver.resolveByBuilderContract (ctor(T), B(), build(), and later the factory methods from #309) currently only verify that the members exist. They should additionally verify the members are accessible from the package the generated builder is placed in — e.g. via ProcessingContext.isMemberAccessibleFromBuilderPackage.

Why

Generated consumer code calls the referenced builder's constructor and build() from the generated builder's package. A candidate whose ctor(T), B() or build() is private — or package-private while the generated builder lands in a different package (e.g. via packageName) — passes the contract today but produces uncompilable code.