Regressions related to #5879

[teunbrand]

This is an issue meant to track fallout from #5879 as determined through reverse dependency check in dbea0a6. Issue #6003 was already identified and fixed in #6004.

Another issue is that names(ggplot()) no longer contains the labels field. This trips up some tests in dependencies that expect that length(ggplot()) is some fixed value. This should be easy to fix.

Another issue is that people sometimes test expect_equal(plot$labels$var, "value"). As plot$labels is no longer filled upon adding layers, plot$labels is empty in many such cases and thus the tests fail. This affects >30 packages and is not straightforward to fix, so we'd need to decide wether to revert #5879 or ask maintainers to adapt their tests.