Apply CL 40930 to src/cmd/gofmt/internal.go to bring
it into sync with src/go/format/internal.go.
Also revert '\n' back to "\n\n" in one of the comments,
because the previous text was more accurate.
Gofmt replaces the "; " part of "package p; func _() {"
input with two newline characters, not one.
Updates #11844
Change-Id: I6bb8155a931b793311991d3cd8e006a2931b167a
Reviewed-on: https://go-review.googlesource.com/c/go/+/221497 Reviewed-by: Robert Griesemer <gri@golang.org>