Move supplementary code into its own crate

[joshwlewis]

This buildpack is currently shipping the inventory management binaries (which are intended for use by GitHub Actions automation only) as part of the published buildpack:

$ docker run --rm -it heroku/builder:22 bash -c 'ls /cnb/buildpacks/heroku_go/*/.libcnb-cargo/additional-bin/'

diff_inventory   update_inventory

This is because currently libcnb-package packages all binaries in the buildpack crate.

This PR moves the inventory management binaries to their own crate, so that these binaries aren't unintentionally included.

Since this PR brings in cargo workspace functionality, it also meant that the buildpack source has changed locations. It's now in ./buildpacks/go, which matches the directory structure of other buildpacks like https://github.com/heroku/buildpacks-nodejs, https://github.com/heroku/buildpacks-ruby, and https://github.com/heroku/buildpacks-jvm.

[edmorley]

Thank you :-)