Refactor libcnb-package and libcnb-cargo in preparation for test support

[colincasey]

The following changes have been made to maximize the sharing of code between libcnb-cargo, libcnb-test, and libcnb-package. This is to prepare for upcoming changes that will allow for testing of local and meta-buildpacks.

[edmorley]

Thank you for splitting out the refactoring from #590 - unfortunately I'm still finding this PR too hard to review as-is, since it's still making a lot of separate changes at once.

Reading the changelog gives a clue that there are actually quite a few parts of this that can be split out, which we can quickly review/merge individually. To speed things up, you could also stack the PRs (ie: set the base branch to the second PR to that of the first, and similar for each PR after that) for any changes that would otherwise result in merge conflicts etc.

[schneems]

In general I agree that features and refactorings should be separate when contributing for review's sake. I also like a tidy(er) git history generally. Also, teasing out this one change into N changes looks like a good bit of time/work. If the main problem is context and the review, one alternative could be to do a pairing session on a review.

I did a pairing session with Colin to walk through changes of the original PR, and that was pretty effective. Maybe you could try that and see what could be merged together and what still needs to be split out?

[colincasey]

got it. i'll resist my tendency to go with more of a shotgun-style approach to refactoring and try to slice these changes into smaller parts 😅

[Malax]

Closing in favour of #657