We're using a distinct Gerrit account to post Kokoro CI results.
Add it to the list of known bots to stop treating it as a human
reviewer.

Also future-proof a little by considering any Gerrit account with the
SERVICE_USER tag to be a bot, since all bot accounts are expected¹ to
be a part of that group.

¹ https://gerrit-review.googlesource.com/Documentation/user-attention-set.html#_important_note_for_all_host_owners_project_owners_and_bot_owners

For golang/go#38906.

Change-Id: If33def5b73fdaadec81b6fdc03a0eaf3042f8095
Reviewed-on: https://go-review.googlesource.com/c/build/+/401514
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

The Go LUCI Gerrit account (https://go-review.googlesource.com/dashboard/60063)
now adds itself as a reviewer or CC on CLs. Even though it as expected
has a SERVICE_USER tag, we still need to add it to a hard-coded list
in humanReviewersOnChange because its first pass uses maintner only,
and tracking Gerrit accounts is not in scope of maintner's corpus.

Also remove the possibility of reviewersInMetas returning the same IDs
more than once, since that doesn't seem helpful, and use slices.Delete
to replace deleteStrings.

Tested with -dry-run -only-run='assign reviewers to CLs' flags: these
changes are enough to get it to add more reviewers to go.dev/cl/534975.

For golang/go#38906.

Change-Id: I1d44292ef56001237030b6b1aa57e65bdd10bf61
Reviewed-on: https://go-review.googlesource.com/c/build/+/534995
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>

CL 534995 added the main Go LUCI Gerrit account, but tricium findings
are reported using https://go-review.googlesource.com/dashboard/62045.

While here, try to reduce the confusion about who "reviewers" includes
by documenting it explicitly. The term is inherently ambiguous because
Gerrit UI displays "Reviewers" and "CC" as two non-overlapping groups,
but in the Gerrit API, "reviewers" includes both groups, relying on
their state being one of "REVIEWER" or "CC" to tell them apart.

Also, reviewersInMetas doesn't deal with detecting humans, so remove
stale mentions of that from its implementation and test code.

For golang/go#38906.

Change-Id: Icde413bb22b20c3336a96195fd097d8456a2ab40
Reviewed-on: https://go-review.googlesource.com/c/build/+/543755
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>