Address all coments in @trws's latest comment in PR #869.

I addressed them by factoring the code better to follow the visitor
pattern.  This will allow actions to be easily added in the future.
These may not even be file sytsem views.  One could add actions to
generate shell init scripts, JSON DAG-dumpers, GraphViz DOT file
generators, etc (yes, some of these are alread in there - just to give
the idea).

Also added is a top-level test

 $ source share/spack/setup-env.sh
 $ ./share/spack/examples/test_view.sh

Read the top of that script first.

var/spack/repos/builtin.mock/packages/test-a/package.py

@@ -9,6 +9,7 @@ class TestA(Package):
     """The test-a package"""
 
     url = 'file://'+source
+    homepage = "http://www.example.com/"
 
     version('0.0', '4e823d0af4154fcf52b75dad41b7fd63')
 

var/spack/repos/builtin.mock/packages/test-b/package.py

@@ -9,6 +9,7 @@ class TestB(Package):
     """The test-b package"""
 
     url = 'file://'+source
+    homepage = "http://www.example.com/"
 
     version('0.0', '4e823d0af4154fcf52b75dad41b7fd63')
 

var/spack/repos/builtin.mock/packages/test-c/package.py

@@ -9,6 +9,7 @@ class TestC(Package):
     """The test-c package"""
 
     url = 'file://'+source
+    homepage = "http://www.example.com/"
 
     version('0.0', '4e823d0af4154fcf52b75dad41b7fd63')
 

var/spack/repos/builtin.mock/packages/test-d/package.py

@@ -9,6 +9,7 @@ class TestD(Package):
     """The test-d package"""
 
     url = 'file://'+source
+    homepage = "http://www.example.com/"
 
     version('0.0', '4e823d0af4154fcf52b75dad41b7fd63')