Fix handling of GitHub URLs

[joshkel]

A package's repository field may contain a Git repository as accessed over https:// or http://. For an example, see @hapijs/inert. (The NPM docs say that the repository field should be usable by the VSC software, not a project homepage; however, GitHub, GitLab, and Bitbucket all support automatically redirecting browsers to the project's homepage if a web browser navigates to this URL.)

Previously, yarn-plugin-outdated would parse repository URLs such as https://github.com/hapijs/inert.git and turn it into https://github.com/https://github.com/hapijs/inert.git, which is incorrect.