@shikokuchuo, I read what you did in the last couple commits, and I really like it. In the future though, it would really help to submit pull requests when we make changes to parts of r-releases, especially this package.

This PR is a refactor and enhancement of URL assertions. In particular:

• It adds more strict checks on the URL, including the use of https and use of a gitlab/github domain.
• It simplifies assert_package() to check the URL and name instead of having to read a text file.
• It renames assert_package_url() to assert_cran_url(), simplifies the code (at least in my eyes), and aligns it to the rest of the package.
• It calls assert_cran_url() from assert_package_url() to make review_pull_request() simpler.

I would strongly prefer to avoid modifying review_pull_request() and review_pull_requests() as much as possible because these functions call the API and are much harder to test. This PR reverts review_pull_request() back to pretty much the way it was, except assert_package_contents() is now replaced by assert_package().