Add a cargo command for performing both packaging and a pack build

[edmorley]

When developing a buildpack there are a few ways to iterate, that all have their place:

1. Via compiler + Clippy + unit test feedback
2. Via libcnb-test powered integration tests
3. Via cargo libcnb package followed by pack build with the correct arguments

Currently (3) requires quite a bit of boilerplate, for example:

cargo libcnb package && pack build --builder heroku/builder:22 --pull-policy if-not-present --path tests/fixtures/empty_procfile/ --buildpack target/buildpack/debug/heroku_procfile procfile-test

In addition, this is also error prone, since it's easy to accidentally point at the wrong compiled buildpack directory (for example release instead of debug), or a leftover previous build (as happened recently with @schneems).

One way to make this workflow easier would be to have a cargo libcnb run (or similar) command, that effectively performed both the cargo libcnb package followed by the pack build with suitable --buildpack arguments. For bonus points this command could clean up the old image (or not, depending on args).

[edmorley]

As an added bonus, if we were to add a cargo libcnb run command, we could then perhaps make cargo libcnb package default to --release (given the primary use-case of that command would then be for production only), thereby fixing #359.