Derive license information from READMEs and LICENSE files

[stephenmathieson]

Several packages are listed as "unlicensed" on their listing, but they either have a LICENSE file, or include their license in their README. We should update the registry builder to check these files before marking a package as unlicensed.

[stephenmathieson]

A quick npm search shows that I've already run into this problem and created a package to solve it 😆