Make it possible to vendor this package

[gridbugs]

This fixes two problems that were preventing this package from working when vendored inside another dune project:

• Replace workspace_root with project_root in dune file. When a package is vendored inside another project, workspace_root is the top-level directory for the entire project whereas project_root is the vendored source directory for this package.

• Pass the path to the cppo binary to discover.exe. If cppo is vendored and not globally installed, dune can locate it (e.g. %{bin:cppo} in a dune file) but running "cppo" won't work.