Related #5795

When using Composer to pull in both WordPress Core and WP-CLI, the static files autoloading that Composer applies to the Requests library causes fatal issues.

This PR extract the Requests library out of the Composer dependencies and it commits it as part of the source. We had already been manually controlling the autoloading of the Requests library to get around the lack of WordPress Core dependency management, so this step only completes the approach we've already been taking.

Note that this is certainly not the correct way of handling a dependency. However, the combination of how WordPress doesn't handle its dependencies and the requirement of WP-CLI to work across all permutations of old/new Core and old/new WP-CLI forces our hand in this case.