Move some content out of deprecated /guides directories

[parlough]

Contributes to #5767 and #4565 by moving a few isolated pages within the /guides directory that have an existing suitable location to there.

These are grouped as an initial step as they don't require larger content or restructuring changes, unlike some other pages within the /guides directory.

Original location	New location
/guides/libraries/useful-libraries	/resources/useful-packages
/guides/libraries/writing-package-pages	/tools/pub/writing-package-pages
/guides/google-apis	/resources/google-apis
/guides/language/coming-from/**	/resources/coming-from/**

[dart-github-bot]

Visit the preview URL for this PR (updated for commit d24e42d):

https://dart-dev--pr5769-misc-deprecated-folder-cleanup-t73ac0l2.web.app

[atsansone]

@parlough : If you aren't getting a 1:1 of issue to PR, consider how you're framing the issue or developing the PR. Highly recommend making either smaller scope issues or completing whole issues in one PR.

[MaryaBelanger]

Agree with @atsansone re:mismatched issue/pr, but I think the problem here is mostly on me for writing these sweeping issues that do need to be broken down to tackle.

I'd say the solution for this kind of thing ("Contributes to #issue"), besides me writing better issues 🙃, would be a little more description in the PR summary about what parts of the mentioned issues this contributes to, how does that sound? That way at least we can go edit the issue description and cross some parts out if they're done. Just a thought!

[parlough]

Thanks for the reviews and feedback!

I didn't see this as an isolated piece of work/issue and it could technically all be in one larger PR later, but just ends up being easier to review in a few small PRs.

I added more details as well as a list of changes to this PRs description and will make sure to do so as well next time :)