Change stdlib imports to use std prefix in most examples (#17202)

lib/system/channels.nim

@@ -30,7 +30,7 @@
 ##   # Be sure to compile with --threads:on.
 ##   # The channels and threads modules are part of system and should not be
 ##   # imported.
-##   import os
+##   import std/os
 ##
 ##   # Channels can either be:
 ##   #  - declared at the module level, or

lib/system/io.nim

@@ -913,7 +913,7 @@ iterator lines*(filename: string): string {.tags: [ReadIOEffect].} =
   ## character(s) are removed from the iterated lines. Example:
   ##
   ## .. code-block:: nim
-  ##   import strutils
+  ##   import std/strutils
   ##
   ##   proc transformLetters(filename: string) =
   ##     var buffer = ""

lib/system/iterators_1.nim

@@ -12,7 +12,7 @@ iterator countdown*[T](a, b: T, step: Positive = 1): T {.inline.} =
   ## **Note**: This fails to count to `low(int)` if T = int for
   ## efficiency reasons.
   runnableExamples:
-    import sugar
+    import std/sugar
     let x = collect(newSeq):
       for i in countdown(7, 3):
         i
@@ -49,7 +49,7 @@ iterator countup*[T](a, b: T, step: Positive = 1): T {.inline.} =
   ## **Note**: This fails to count to `high(int)` if T = int for
   ## efficiency reasons.
   runnableExamples:
-    import sugar
+    import std/sugar
     let x = collect(newSeq):
       for i in countup(3, 7):
         i
@@ -78,7 +78,7 @@ iterator `..`*[T](a, b: T): T {.inline.} =
   ## See also:
   ## * [..<](#..<.i,T,T)
   runnableExamples:
-    import sugar
+    import std/sugar
 
     let x = collect(newSeq):
       for i in 3 .. 7:

lib/system/threads.nim

@@ -24,7 +24,7 @@
 ##
 ## .. code-block:: Nim
 ##
-##  import locks
+##  import std/locks
 ##
 ##  var
 ##    thr: array[0..4, Thread[tuple[a,b: int]]]