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-vegan/package.py

@@ -31,6 +31,7 @@ class RVegan(RPackage):
 
     homepage = "https://github.com/vegandevs/vegan"
     url      = "https://cran.r-project.org/src/contrib/vegan_2.4-3.tar.gz"
+    list_url = "https://cran.r-project.org/src/contrib/Archive/vegan"
 
     version('2.4-3', 'db17d4c4b9a4d421246abd5b36b00fec')