system.nim cleanup some exported constants which should never have be… (#17909)

* system.nim cleanup some exported constants which should never have been exported

lib/pure/coro.nim

@@ -21,6 +21,8 @@
 ##
 ## Unstable API.
 
+import system/coro_detection
+
 when not nimCoroutines and not defined(nimdoc):
   when defined(noNimCoroutines):
     {.error: "Coroutines can not be used with -d:noNimCoroutines".}