code: def R_package_path(package):
      
      local_sitelib = R_sitelib()
      rscript = Rscript_cmd()
      cmd = """{rscript} --no-environ -e '.libPaths(c("{local_sitelib}")); find.package("{package}")'"""
      try:
          output = subprocess.check_output(cmd.format(**locals()), shell=True)
      except subprocess.CalledProcessError as e:
          return None
      for line in output.decode().split("\n"):
          if "[1]" not in line:
              continue
          dirname = line.split("[1]")[1].replace("\"", "").strip()
          if os.path.exists(dirname):
              return dirname
      return None
docs: return the path to an installed R package