(Needed so that I can vendor this into std,
a consequence of vendoring x/tools into std.)

A number of places were further simplified by hand.

Change-Id: Ic836afb1623365edcf81f5363f213366a2978d28
Reviewed-on: https://go-review.googlesource.com/c/mod/+/720581
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

CL 720581 replaced almost all instances of it, this gets the last one.

[git-generate]
cd sumdb/note
rf '
	ex { import "strings"; chop -> strings.Cut }
	rm chop
'

Change-Id: I3bd4665c959c7b6fd2729189a6e7cfb101e5a4b7
Reviewed-on: https://go-review.googlesource.com/c/mod/+/720700
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Update golang.org/x dependencies to their latest tagged versions.

Change-Id: I56eac9edfc36572201aacff64c3568b06eaa6d10
Reviewed-on: https://go-review.googlesource.com/c/mod/+/728000
Auto-Submit: Gopher Robot <gobot@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: David Chase <drchase@google.com>