Testing the plot$labels field

[teunbrand]

This problem was identified in #6287 and affects 32 packages.

Many packages use the labels plot field in their tests to see if their code has correctly constructed their plot.
We now only populate this field with the labs() function or when the plot is build, and no longer when layers are added automatically.
We have an accessor in place that people can use (#6078).

Should we start sending out PRs to affected packages?

[teunbrand]

TODO: Send out PRs to packages

[teunbrand]

A suggestion was raised in tidyverts/feasts#166 (comment).
Essentially, annotate() populates labels and it would be nice if these were deprioritised in favour of regular layers.

[teunbrand]

For the 32 affected packages, I have taken the following actions:

• I've sent out 28 PRs with suggested changes to tests.
• I've sent out 3 emails to maintainers whose packages aren't on GitHub, warning them of this change.
• I've filed 1 issue where a fix was not straightforward due to off-label use.

That being done, I think this issue should be considered resolved to the best of ggplot2s capabilities and the only thing left to do for me is to shepard the open PRs.