[build2] publishing packages with external tests

[Boris Kolpackov]

Boris Kolpackov <boris at codesynthesis.com> writes:

> But I can see how this arrangement clashes with the above recommendation
> to exclude tests with stubbed dependencies from the build since now the
> only place this can be done is the package itself, but that would also
> exclude it from the builds, which we don't want. Hm, let me meditate on
> this and get back to you.

Ok, in light of this for the next release we've changed the semantics in
this area. Specifically:

1. Standalone builds for a separate test package that is referenced
   as tests/examples/benchmarks from the primary package are now
   automatically omitted. This was previously achieved with an
   explicit builds:none.

2. The builds and build-include/exclude values in such a separate
   test package now control building of this package as part of
   the primary package. In other words, such a test package can
   now exclude itself from additional (with regards to the primary
   package) build configurations.