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".}

lib/pure/os.nim

@@ -1910,7 +1910,7 @@ proc copyFileToDir*(source, dir: string, options = {cfSymlinkFollow})
   copyFile(source, dir / source.lastPathPart, options)
 
 when not declared(ENOENT) and not defined(windows):
-  when NoFakeVars:
+  when defined(nimscript):
     when not defined(haiku):
       const ENOENT = cint(2) # 2 on most systems including Solaris
     else: