Error or warning for packages without a valid license field

[s-ludwig]

It's crucial to know which licenses are in the game when using foreign projects. It should be enforced/encouraged to have this information available for every package in the registry.

If only warnings are used, displaying the license as "proprietary" and making it incompatible with every other package so that dub warns about it (dlang/dub#117) is probably the way to go.