Verify that the referred repository indeed contains the code of the crate

[szabgab]

As the "repository" field is free text field it is quite easy by mistake or on purpose to link to the repository of another project. If we assume blindly that the repository link is correct and we collect data from the repository we can be fooled (e.g. to think that a crate has a lot of stars on GitHub even though we are looking at a different repo.)