Add latest package size to spk report output

[dcookspi]

A user requested we add the (latest version) package size to spk report output when running it on a package.

[rydrman]

From the meeting today:

• will this slow everything else down if it means that the command now has to load the manifests in addition to the packages?
• if so, this could also just be an opt-in flag
• the actual use case for this is not entirely clear, would be good to write down as part of this design if there's a strong one