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-gmodels/package.py

@@ -30,6 +30,7 @@ class RGmodels(RPackage):
 
     homepage = "http://www.sf.net/projects/r-gregmisc"
     url      = "https://cran.r-project.org/src/contrib/gmodels_2.16.2.tar.gz"
+    list_url = "https://cran.r-project.org/src/contrib/Archive/gmodels"
 
     version('2.16.2', 'f13e5feb2a8b9f0cd47fdf25ddc74228')