add new validation: `repos/archive` contains no files

[marcoieni]

Discussed in #1527 (comment)

I think there are two ways of doing this:

• add the repo path to the struct Repo contained in the struct Data. In this way we can easily validate that the path is not under repos/archive in the check added in validate archived repos #1527
• edit the load function and check that all entries of the archive directory aren't files.

I prefer option 1 because it's cleaner and doesn't complicate the "load" method. What do you think?

Unfortunately we can't edit the closure that we pass to load_dir because it only runs on repos under the subdirectories of archive, e.g. repos/archive/rust-lang.

[Kobzol]

I was hoping that we could just modify the existing validations, but I see that they work on Data, which doesn't know about the filesystem anymore. I don't like option 1), because it would introduce an external information (filesystem path) into the Data world, which otherwise doesn't deal with the filesystem.

But I also don't think that we need to modify the load closure. Instead, we could just return an error here.

[marcoieni]

Great suggestion! done #1530