Instead of a fake build dependency, move the links key from pyo3 to pyo3-ffi
which according to [1] is a prerequisite for build scripts to emit meta-data
that can be used by other build scripts which would incidentally ensure the
ordering between the build script of pyo3 and pyo3-ffi that we need.
[1] https://doc.rust-lang.org/cargo/reference/build-script-examples.html#using-another-sys-crate
