x/tools/internal/stdlib: update stdlib index after every major Go release

[smasher164]

API changes should automatically be detected by x/tools/internal/imports after each major release. After 1.14, they were manually updated as per #38464. This should be part of the release process (maybe x/build/cmd/releasebot?).

[gopherbot]

Change https://golang.org/cl/246581 mentions this issue: internal/imports: update stdlib index for 1.15

[heschi]

@golang/release might have something to say about this.

[gopherbot]

Change https://golang.org/cl/293839 mentions this issue: internal/imports: update stdlib index for 1.16

[dmitshur]

As a first step, let's make this a recurring early-in-cycle release-blocking issue, similar to #40705 and #36905, so that this task isn't missed.

(If it's viable to automate this task so it doesn't need to be done by hand every 6 months, please file a new issue for doing that work.)

I'll place it in Go 1.17 milestone now, and when CL 293839 is submitted, it can be moved to Go 1.18.