Use explicit imports from plyr

[MichaelChirico]

Related to #1382, #1383 -- by making it clearer which names come from {plyr}, we make it easier to mark out progress on dropping {plyr}.

[MichaelChirico]

FWIW, I just saw that the models object in inst/models/models.RData also includes "plyr" among several models${MODEL}$library such as models$gbm$library:

names(which(sapply(models, \(x) "plyr" %in% x$library)))
 [1] "ada"              "AdaBag"           "AdaBoost.M1"      "blackboost"       "BstLm"            "bstSm"            "bstTree"          "C5.0"             "C5.0Cost"        
[10] "gamboost"         "gbm"              "glmboost"         "mlpSGD"           "ordinalNet"       "PenalizedLDA"     "relaxo"           "rfRules"          "rotationForestCp"
[19] "rpartCost"        "rpartScore"       "spikeslab"        "treebag"          "xgbDART"          "xgbTree"

I haven't noticed any tests failing until now; only by removing {plyr} from {reshape2} did I start seeing failures in {posterior} from this:

https://github.com/stan-dev/posterior/blob/3e5368b97f270dca65ff54120016f0f50b3e2105/tests/testthat/test-rstar.R#L34