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/hue/bridge.py

@@ -51,7 +51,8 @@ class HueBridge:
             # linking procedure. When linking succeeds, it will remove the
             # old config entry.
             hass.async_add_job(hass.config_entries.flow.async_init(
-                DOMAIN, source='import', data={
+                DOMAIN, context={'source': config_entries.SOURCE_IMPORT},
+                data={
                     'host': host,
                 }
             ))