get all imported R packages

#---------------------------
# remove all existent objects
#---------------------------
rm(list = ls())
gc()

#---------------------------
# installation of all required packages
#---------------------------
required_pkgs <- c("httr", "xml2", "pipeR", "stringr")
if (any(!required_pkgs %in% rownames(installed.packages())))
  install.packages(required_pkgs[!required_pkgs %in% rownames(installed.packages())])

#---------------------------
# library all required packages
#---------------------------
invisible(sapply(required_pkgs, library, character.only = TRUE))

#---------------------------
# capture names of all packages
#---------------------------
cran <- "https://cran.r-project.org/web/packages/available_packages_by_date.html"
all_pkg_a <- GET(cran) %>>% content %>>% xml_find_all("//td/a")
pkg_names <- xml_text(all_pkg_a)

#---------------------------
# create urls for searching each package
#---------------------------
urls <- paste(dirname(cran), xml_attr(all_pkg_a, "href"), sep = "/")

#---------------------------
# get all imports info and spilt the package name of imported packages
#---------------------------
library(parallel)
cl <- makeCluster(4L)
clusterExport(cl, "required_pkgs")
clusterEvalQ(cl, sapply(required_pkgs, library, character.only = TRUE)) %>>% invisible
t1 <- proc.time()
package_crawler <- clusterApplyLB(cl, urls, function(url){
  pkg_imports <- GET(url) %>>% content %>>% xml_find_all("//td[contains(., 'Imports')]") %>>%
    xml_parent %>>% xml_find_all("td") %>>% `[[`(2) %>>% xml_text("text")
  str_match_all(pkg_imports, "([a-zA-Z0-9]+)(\\s+\\([^,]+\\))?")[[1]]
})
proc.time() - t1
stopCluster(cl)