stacks: update view management for multiple/combinatorial views

This adds notion of a default view, other views in environments

var/spack/repos/builtin.mock/packages/mpich/package.py

@@ -28,4 +28,4 @@ class Mpich(Package):
     provides('mpi@:1', when='@:1')
 
     def install(self, spec, prefix):
-        pass
+        touch(prefix.mpich)

var/spack/repos/builtin.mock/packages/mpileaks/package.py

@@ -27,7 +27,7 @@ class Mpileaks(Package):
     libs = None
 
     def install(self, spec, prefix):
-        pass
+        touch(prefix.mpileaks)
 
     def setup_environment(self, senv, renv):
         renv.set('FOOBAR', self.name)