We also set `resolver = "2"` on the workspace; this is the default for
the root package in Rust 2021, but as we use a virtual workspace we need
to explicitly set it instead.
The published source code for each package needs to include the required
header file, and the path to that header file needs to be relative to
the package source (not the repository source). We therefore need to
have the header file present in each workspace package.
Closeszcash/halo2#506.