Add more loader dunders to cp module.

Using cp module to dip our feet into more explicit loader dunders. Most
all of the existing dunders can be imported already. The one outlier
being used by the cp module is __opts__.

salt/loader/dunder.py

@@ -8,3 +8,6 @@
 
 
 __file_client__ = loader_context.named_context("__file_client__", default=None)
+__context__ = loader_context.named_context("__context__")
+__pillar__ = loader_context.named_context("__pillar__")
+__grains__ = loader_context.named_context("__grains__")

salt/modules/cp.py

@@ -20,7 +20,7 @@
 import salt.utils.templates
 import salt.utils.url
 from salt.exceptions import CommandExecutionError
-from salt.loader.dunder import __file_client__
+from salt.loader.dunder import __context__, __file_client__, __grains__, __pillar__
 
 log = logging.getLogger(__name__)