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-assertthat/package.py

@@ -36,7 +36,7 @@ class RAssertthat(Package):
 
     version('0.1', '59f9d7f7c00077ea54d763b78eeb5798')
 
-    extends('R')
+    extends('r')
 
     def install(self, spec, prefix):
         R('CMD', 'INSTALL', '--library={0}'.format(self.module.r_lib_dir),