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/error.rs

@@ -147,9 +147,7 @@ impl From<BuildBinariesError> for Error {
                 BuildError::UnexpectedCargoExitStatus(exit_status),
             ) => Error::BinaryBuildExitStatus {
                 target,
-                code: exit_status
-                    .code()
-                    .map_or_else(|| String::from("<unknown>"), |code| code.to_string()),
+                code: exit_status_or_unknown(exit_status),
             },
 
             BuildBinariesError::MissingBuildpackTarget(target) => {