Cleanup index initialization

[Jake-Shadle]

• Disable rustfmt
• Add CRATES_IO_HTTP_INDEX
• Cleanup index initialization

This exposes a new get_index_details function that takes a url and turns it into the disk location, rooted at the optional location or CARGO_HOME/cargo default, as well as the canonicalized url. This also adds SparseIndex::with_path which allows a user to easily create a sparse index at the specific location but still with the same layout as cargo and not be dependent on CARGO_HOME. This meant sparse_index::test::parses_cache could be drastically simplified to remove the mucking about with environment variables.

[kornelski]

Thank you for splitting the PR. That went much smoother.