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/isaac/package.py

@@ -31,10 +31,6 @@ class Isaac(CMakePackage):
     homepage = "http://computationalradiationphysics.github.io/isaac/"
     url      = "https://github.com/ComputationalRadiationPhysics/isaac/archive/v1.3.0.tar.gz"
 
-    @property
-    def root_cmakelists_dir(self):
-        return join_path(self.stage.source_path, 'lib')
-
     version('develop', branch='dev',
             git='https://github.com/ComputationalRadiationPhysics/isaac.git')
     version('master', branch='master',
@@ -55,3 +51,5 @@ def root_cmakelists_dir(self):
     # depends_on('alpaka', when='+alpaka')
     depends_on('icet', type='link')
     depends_on('mpi', type='link')
+
+    root_cmakelists_dir = 'lib'