Better support for package.json's repository field

Currently it only works if it's in the "username/reponame" format, but I'd like to support the same things that npm does https://docs.npmjs.com/files/package.json#repository
(this is for producing the "edit" links on each page)

Start by locating the package.json repository-field handling and the code that produces edit links for generated pages. Compare the accepted formats with npm's repository documentation, then verify that each supported form produces the expected edit link.

Written by the indexing model from the issue text.