bugfix: install --only dependencies works in env (#13090)

* bugfix: install --only dependents works in env

includes regression testing

var/spack/repos/builtin.mock/packages/dependent-install/package.py

@@ -13,8 +13,10 @@ class DependentInstall(Package):
     url      = "http://www.example.com/a-1.0.tar.gz"
 
     version('1.0', '0123456789abcdef0123456789abcdef')
+    version('2.0', '0123456789abcdef0123456789abcdef')
 
-    depends_on('dependency-install')
+    depends_on('dependency-install@2.0', when='@2.0')
+    depends_on('dependency-install@1.0', when='@1.0')
 
     def install(self, spec, prefix):
         touch(join_path(prefix, 'an_installation_file'))