Support default not null annotations

[Katrix]

Some libraries that never return null instead opt for a package level default not null annotation, which marks everything as not null. Would be neat if Dotty's explicit nulls would support that.

Definition: https://github.com/SpongePowered/SpongeAPI/blob/stable-7/src/main/java/org/spongepowered/api/util/annotation/NonnullByDefault.java Usage: https://github.com/SpongePowered/SpongeAPI/blob/stable-7/src/main/java/org/spongepowered/api/package-info.java