How to find out which modules have brought in a particular module

[Gavin Smith]

I'm interested in reducing the number of checks done in a configure
script: one way could be to make more use of conditional dependencies
between modules. gnulib-tool --add-import lists the modules which were
used and which were brought in as dependencies. However, there are
some things I'd like to do that I couldn't find options for. One is to
see which modules brought in a module. The way I was doing this was by
going into the "gnulib/modules" directory in the Gnulib checkout and
grepping for the name of the module. It would be nice to be able to
get this information automatically, and in a way that takes into
account indirect dependencies. Maybe there should be an option for
gnulib-tool that can list the modules that have been brought in, and
for each of them, list the modules that have been explicitly asked for
that depend on the module, directly or indirectly, and also list the
modules that depend on the module conditionally.

For example I wondered why the checks for the unistd module were being
run. I found that getopt-posix had a dependency on it. I edited the
module file to make this a conditional dependency, reran gnulib-tool
--add-import, ran "make configure" to remake the configure script: but
when I ran "configure" again, the tests were still run. I expected
that there was another module also depending on unistd, but it wasn't
immediately obvious which one, because several modules that could have
been imported depended on unistd. That's as far as I got investigating
the matter. Is there an easier way to investigate this kind of thing,
that I've been missing? Or would gnulib-tool benefit from the extra
functionality I've suggested?