Improve PackageBuilder

[oscartbeaumont]

Changes:

• Ensure optional field are not required using builder(default). The documentation implies this can be put on the struct instead of each field but in practice that requires Package: Default which we probally don't want.
• Add a PackageBuilder::new method which takes all required fields and returns a PackageBuilder. This is optional as PackageBuilder::default does work it provides some extra safety to downstream users as they will get compile errors if required fields are added in a future major.

One implication of adding PackageBuilder::new is that adding a new required field will requiring modifying the new method which is a breaking change requiring a major version. Technically in the old setup you could be add a non-required field without breaking the compliation of downstream crates due to non_exhaustive but in practice it would have been a breaking change to the runtime behavior so it would never have been okay anyway.

Closes my comments in #158.