imagine the following scenario:
- you make a change in module SVX
- you make depending changes in selected modules depending on SVX
- you add SVX and the selected modules to your source_config file
- you want to build SVX, and the selected modules, with a single command

Normally, I'd expect "build --from svx" to do this. However, if you attempt
that, then build.pl will complain, because there are modules depending on SVX
which are *not* part of source_config.

That's somewhat counter-intuitive, in my opinion. In other modes (e.g. build
--all), source_config is some kind of additional layer, which comes into play
right after build.pl calculated the modules to build. That is, *first* build.pl
determines the modules which it should build, *then* it consults source_config
to restrict this module list further.

So, the request would be to let "build --from" behave this way, too - use
source_config as final instance to limit the to-be-built modules, instead of
bailing out with an error.

Of course, the situation might still warrant a warning, issued by build.pl.