Using cargo vendored directory sources for crate_universe?

[afq984]

We have rust libraries vendored at https://chromium.googlesource.com/chromiumos/third_party/rust_crates/ and I'm wondering if it's possible to make crate_universe use it, instead of crates.io.

With cargo, it is possible to set a directory source replacement but if I feed the cargo config to crates_repository I get an error saying Failed to load index for url: https://github.com/rust-lang/crates.io-index

A reproducer is at https://github.com/afq984/directory-source