[build2] Issues with my first attempt at build2 CI

[Klaim - Joël Lamotte]

I think I found the source of this issue: in gtest-main.cc (the file
being processed when the error occurs) there is the following:

    // The following lines pull in the real gtest *.cc files.
    #include "src/gtest.cc"
    #include "src/gtest-death-test.cc"
    #include "src/gtest-filepath.cc"
    #include "src/gtest-port.cc"
    #include "src/gtest-printers.cc"
    #include "src/gtest-test-part.cc"
    #include "src/gtest-typed-test.cc"

Apparently bpkg didn't exactly clone gtest repo as a submodule but
instead retrieved the files it needed that were mentionned in the
buildfile, not the others which for my case are missing.

1. Am I correct that this is the intended behavior of bpkg?
2. If yes, the fix is to add these files (in some ways) explicitely as
dependencies of the target in question. Am I correct? Or should I
approach this differently?

Joël