Add find_cargo_workspace utility method

In preparation for adding support for testing meta-buildpacks the `find_cargo_workspace` method is being added to `libcnb-package` so that it can be shared between `libcnb-package` and `libcnb-test`.

Both the packaging and testing processes will use this to locate buildpacks within a workspace in order to resolve project-level dependencies declared with `libcnb:{buildpack_id}`.

Author: colincasey

libcnb-cargo/src/package/command.rs

@@ -33,7 +33,6 @@ pub(crate) fn execute(args: &PackageArgs) -> Result<()> {
         .map_err(|e| Error::FindBuildpackDirs(workspace_dir.clone(), e))?;
 
     let buildpack_packages = buildpack_dirs
-        .into_iter()
         .into_iter()
         .map(read_buildpack_package)
         .collect::<std::result::Result<Vec<_>, _>>()?;