Function update_crates_test_bin::index_subpath

pub(crate) fn index_subpath(package_name: &str) -> PathBuf

from https://doc.rust-lang.org/cargo/reference/registries.html:

The rest of the index repository contains one file for each package, where the filename is the name of the package in lowercase. Each version of the package has a separate line in the file. The files are organized in a tier of directories:

• Packages with 1 character names are placed in a directory named 1.
• Packages with 2 character names are placed in a directory named 2.
• Packages with 3 character names are placed in the directory 3/{first-character} where {first-character} is the first character of the package name.
• All other packages are stored in directories named {first-two}/{second-two} where the top directory is the first two characters of the package name, and the next subdirectory is the third and fourth characters of the package name. For example, cargo would be stored in a file named ca/rg/cargo.