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's not entirely clear what subnode ranges should mean in the
presence of macros, so let's leave them out for now. We are not using
them heavily anyway.
