automake/rust: remove path.lib

automake/rust: remove path.lib

Remove the path.lib parameter that is substituted into the output
Cargo.toml by autoconf. Instead, as part of the build, "cd" into the
source directory. We already set the Rust target directory to the
external build directory.

This makes the Cargo.toml more generic, and in a format suitable for
publishing to crates.io. It also makes it easier to pull in external
crates without needing to patch up their Cargo.toml, for example, it
might make pulling libhtp-rs easier.