Rename initPathExts to pathExts, make it return the slice of extensions,
and wrap into sync.OnceValue.
While at it, return early if PATHEXT is empty.
Change-Id: I33508762e87edd226e0a52df4063473c496c0210
Reviewed-on: https://go-review.googlesource.com/c/go/+/611017
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Ian Lance Taylor <iant@google.com> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>