Detect module flags by walking package.el files.
This still does not build all dependencies, because some are enabled
only if some flag or other module is disabled. But this should be close.
...not yet with all flags, but that's the next step.
This surfaced several problems not caught by the existing "full" build,
because that did not enable dependencies conditionally enabled if a
second module is enabled.
There are a small number of dependencies only enabled if a second module
is *not* enabled, which I intend to add some manual tests for.