Re-add list_url for all R packages (#6110)

* r-igraph: find old packages in the archive

Fixes #6104.

* r-irlba: find old packages in the archive

Fixes #6101.

* r-lazyeval: find old packages in the archive

Fixes #6102.

* packages: set list_url for all packages hosted on CRAN

* packages: consistently use the CRAN archive URLs

var/spack/repos/builtin/packages/r-xtable/package.py

@@ -30,5 +30,6 @@ class RXtable(RPackage):
 
     homepage = "http://xtable.r-forge.r-project.org/"
     url      = "https://cran.r-project.org/src/contrib/xtable_1.8-2.tar.gz"
+    list_url = "https://cran.r-project.org/src/contrib/Archive/xtable"
 
     version('1.8-2', '239e4825cd046156a67efae3aac01d86')