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

lib/posix/posix.nim

@@ -1089,7 +1089,7 @@ template onSignal*(signals: varargs[cint], body: untyped) =
   ## Example:
   ##
   ## .. code-block::
-  ##   from posix import SIGINT, SIGTERM, onSignal
+  ##   from std/posix import SIGINT, SIGTERM, onSignal
   ##   onSignal(SIGINT, SIGTERM):
   ##     echo "bye from signal ", sig
 

lib/posix/posix_utils.nim

@@ -117,7 +117,7 @@ proc osReleaseFile*(): Config {.since: (1, 5).} =
   ## `os-release` file is not available on Windows and OS X by design.
   ## * https://www.freedesktop.org/software/systemd/man/os-release.html
   runnableExamples:
-    import parsecfg
+    import std/parsecfg
     when defined(linux):
       let data = osReleaseFile()
       echo "OS name: ", data.getSectionValue("", "NAME") ## the data is up to each distro.