Now, when you selectively enable logs from particular crates with
`RUST_LOG=call,worktree`, logs created via `log_err` calls in those
crates get correctly enabled. Previously, they were all attributed to
the `util` crate, because they used the normal logging macros, which
implicitly insert the current crate name.

This relies on the regularity of our directory naming. Rust's
`track_caller` feature allows you to obtain the file system path of the
caller, but not its rust module path, so I'm inferring the crate name
from the file system path (which I believe is always valid, in our
codebase).

Release Notes:

- N/A