go.mod: upgrade to Go 1.17

This change was produced using 'go mod tidy -go=1.17'
with a go command built at CL 315210.

This activates lazy loading for the x/tools module, and updates the
go.mod file to maintain the lazy-loading invariants (namely, including
an explicit requirement for every package transitively imported by the
main module).

I will send a separate CL to upgrade the gopls module, since it has
some awkward interactions with the Go 1.12 presubmit test
(golang/go#45919).

Note that this does *not* prevent users with earlier go versions from
successfully building packages from this module.

For golang/go#36460.

Change-Id: I1d2a6b28820fa6cb3d02162c14873643e4c689cf
Reviewed-on: https://go-review.googlesource.com/c/tools/+/315570
Trust: Bryan C. Mills <[email protected]>
Run-TryBot: Bryan C. Mills <[email protected]>
gopls-CI: kokoro <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Alexander Rakoczy <[email protected]>

Author: Bryan C. Mills

go.mod

@@ -1,6 +1,6 @@
 module golang.org/x/tools
 
-go 1.16
+go 1.17
 
 require (
 	github.com/yuin/goldmark v1.3.3