cmd/dist: add -json flag
This enables JSON output for all tests run by dist.
Most the complexity here is that, in order to disambiguate JSON
records from different package variants, we have to rewrite the JSON
stream on the fly to include variant information. We do this by
rewriting the Package field to be pkg:variant so existing CI systems
will naturally pick up the disambiguated test name.
Fixes #37486.
Change-Id: I0094e5e27b3a02ffc108534b8258c699ed8c3b87
Reviewed-on: https://go-review.googlesource.com/c/go/+/494958
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>