NimScript: prepare for future Nimble improvements; should be completely compatible with old and upcoming Nimble versions

lib/system/nimscript.nim

@@ -293,6 +293,8 @@ template task*(name: untyped; description: string; body: untyped): untyped =
     setCommand "nop"
     `name Task`()
 
+when not defined(nimble):
+  # nimble has its own implementation for these things.
   var
     packageName* = ""    ## Nimble support: Set this to the package name. It
                          ## is usually not required to do that, nims' filename is