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/r-xlsx/package.py

@@ -35,7 +35,7 @@ class RXlsx(Package):
 
     version('0.5.7', '36b1b16f29c54b6089b1dae923180dd5')
 
-    extends('R')
+    extends('r')
 
     depends_on('r-rjava', type=nolink)
     depends_on('r-xlsxjars', type=nolink)