[build2] Issues with my first attempt at build2 CI

[Klaim - Joël Lamotte]

Hi,
my gtest conversion to build2 attempt is close to something usable
(some tests seem to fail but apparently it's expected, because it's
testing the failure - I didn't handle it with testscript yet).
(Also GMock is not setup yet)

I wanted to try the new CI service and commands but got some build
issues as you can see there:
https://ci.cppget.org/@47749680-94fe-4507-8222-92fd1d286e6e/gtest/1.8.0-a.1

The error is on attempting update:
    error: no rule to update
gtest-1.8.0-a.1/gtest/gtest-origin/googletest/src/cxx{gtest.cc}

Here gtest-origin/ is a git submodule of github's gtest, set to v1.8.0 tag.

I'm failing to see what I'm doing wrong:
 1. it's building for me on windows+cl (but let's ignore that of course);
 2. I correctly wrote in root.buid:
    using cxx
    hxx{*}: extension = h
    cxx{*}: extension = cc
because that's what gtest uses (and it compiles as expected locally).
 3. Is this because of the submodule not being initialized?
  That would explain it. But if I remember correctly, bpkg add
<git-url> should handle this and fetch+init the submodules too, right?
That's what is used to fetch my project in the build log.

Anyway I'm not sure what to do to fix this. Any help would be welcome.

Suggestion: in the log, display the build2's version being used.

Joël