Stop doiing directory listings on backpan.metacpan.org

[ranguard]

We are moving https://backpan.metacpan.org to an object store backend and we do not want to implement directory listings due to:

• Additional complexity (we'd have to have different authentication and then something to build the index)
• Cost - making API calls to list directories, however small could add up when we have 14.5k author directories and would want to keep the caching small
• Bot feeding - we don't need the bots eating from us and dir indexes feed them.
• All information can be found on https://metacpan.org/ OR https://www.cpan.org/ (If you want to see if an author has a top level README file for example)

Turning them off now to see if any unthought of impact