Incorrect module paths for files not in an crate/src/ directory

[rozbb]

siderophile naively looks for "src" occurring in the current Rust file path, and will sometimes overshoot the actual crate root. This sometimes gives output like

unsafe expr in function log::github::log_0::tests::filters::set_boxed_logger

since filters.rs presumably occurs in the tests/ directory of the log crate.

A neat fix would be to give a relative path to ast_walker::find_unsafe_in_file instead of an absolute path.