Standardize package names: lower-case, not Mixed_CASE (#2475)

* Rename packages

* Upcasing depends_on() in packages.

* Downcased extends('r')

* Fixed erroneously changed URL that had slipped through.

* Fixed typo

* Fixed link from documentation into package source code.

* Fixed another doc problem.

* Changed underscores to dashes in package names.

* Added test to enforce lowercase, no-underscore naming convention.

* Fix r-xgboost

* Downcase more instances of 'R' in package auto-creation.

* Fix test.

* Converted unit test packages to use dashes not underscores

* Downcase `r` in the docs.

* Update module_file_support.rst

Fix r->R for class R.

var/spack/repos/builtin/packages/julia/package.py

@@ -222,7 +222,7 @@ def install(self, spec, prefix):
         if "+plot" in spec:
             julia("-e", 'Pkg.add("PyPlot"); using PyPlot')
             julia("-e", 'Pkg.add("Colors"); using Colors')
-            # These require maybe Gtk and ImageMagick
+            # These require maybe gtk and image-magick
             julia("-e", 'Pkg.add("Plots"); using Plots')
             julia("-e", 'Pkg.add("PlotRecipes"); using PlotRecipes')
             julia("-e", 'Pkg.add("UnicodePlots"); using UnicodePlots')