go.mod: upgrade to Go 1.17
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 <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Run-TryBot: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools