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/quinoa/package.py

@@ -55,7 +55,7 @@ class Quinoa(CMakePackage):
     depends_on("pstreams")
     depends_on("pegtl")
 
-    root_cmakelists_dir = '../src'
+    root_cmakelists_dir = 'src'
 
     def build_type(self):
         spec = self.spec