rust-project.json: Generate proc-macro paths

`rust-analyzer` can expand and reason about code if paths to the built
proc macro files are provided. Generate these from Soong.

Bug: 372634041
Test: Check that rust-project.json contains proc-macro paths
Test: Open system/security/mls/mls-rs-crypto-boringssl/src/lib.rs (for #[derive(Error)])
Change-Id: Ie0985540f1efdf1bbce6a7433ec414bdcad45ea3