[cover] move time_based_cover to its own subdirectory (#15313)

Co-authored-by: Rene <rene@guca.at>

esphome/components/time_based/__init__.py

@@ -0,0 +1,3 @@
+import esphome.codegen as cg
+
+time_based_ns = cg.esphome_ns.namespace("time_based")

esphome/components/time_based/cover/__init__.py

@@ -11,7 +11,8 @@ from esphome.const import (
     CONF_STOP_ACTION,
 )
 
-time_based_ns = cg.esphome_ns.namespace("time_based")
+from .. import time_based_ns
+
 TimeBasedCover = time_based_ns.class_("TimeBasedCover", cover.Cover, cg.Component)
 
 CONF_HAS_BUILT_IN_ENDSTOP = "has_built_in_endstop"