Anchoring to the SourceRoot wont' work if the path is absolute:
#[path = "/tmp/foo.rs"]
mod foo;
Anchoring to a file will.
However, we *should* anchor, instead of just producing an abs path.
I can imagine a situation where, for example, rust-analyzer processes
crates from different machines (or, for example, from in-memory git
branch), where the same absolute path in different crates might refer
to different files in the end!
It improves compile time in `--release` mode quite a bit, it doesn't
really slow things down and, conceptually, it seems closer to what we
want the physical architecture to look like (we don't want to
monomorphise EVERYTHING in a single leaf crate).
