10385: Make `extern crate test;` work r=jonas-schievink a=jonas-schievink
This implements support for dependencies that are not added to the extern prelude of a crate, and add the `test` crate from the sysroot as such a dependency.
This does mean we now index `test` on startup, but I didn't notice much of a difference (and also, r-a can be used while it is still indexing).
Fixes https://github.com/rust-analyzer/rust-analyzer/issues/6714
bors r+
Co-authored-by: Jonas Schievink <jonasschievink@gmail.com>
1bd14e0530