Refactor data entry flow (#15883)

* Refactoring data_entry_flow and config_entry_flow

Move SOURCE_* to config_entries
Change data_entry_flow.FlowManager.async_init() source param default
 to None
Change this first step_id as source or init if source is None
_BaseFlowManagerView pass in SOURCE_USER as default source

* First step of data entry flow decided by _async_create_flow() now

* Lint

* Change helpers.config_entry_flow.DiscoveryFlowHandler default step

* Change FlowManager.async_init source param to context dict param

homeassistant/components/nest/config_flow.py

@@ -58,6 +58,10 @@ class NestFlowHandler(data_entry_flow.FlowHandler):
         """Initialize the Nest config flow."""
         self.flow_impl = None
 
+    async def async_step_user(self, user_input=None):
+        """Handle a flow initialized by the user."""
+        return await self.async_step_init(user_input)
+
     async def async_step_init(self, user_input=None):
         """Handle a flow start."""
         flows = self.hass.data.get(DATA_FLOW_IMPL, {})