Change path to CMakeLists.txt to be relative to root, not pwd (#4420)

* Change path to CMakeLists.txt to be relative to root, not pwd

* Changes requested during code review

* Revert back to old naming of root_cmakelists_dir
* Make relative directory more clear in docs

* Revert change causing build_type AttributeError

* Fix forgotten abs_path var

* Update CLHEP with new relative path

* Update more packages with new root_cmakelists_dir syntax

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

@@ -63,7 +63,7 @@ def patch(self):
                     '%s/%s/CLHEP/CMakeLists.txt'
                     % (self.stage.path, self.spec.version))
 
-    root_cmakelists_dir = '../CLHEP'
+    root_cmakelists_dir = 'CLHEP'
 
     def build_type(self):
         spec = self.spec