have a separate build target to download any missing deps but not to compile or install them

[zooko]

As mentioned by gdt in comment:9:ticket:1200, it would be useful for packagers if they could ask the build system to download any missing dependencies, but not to build them or install them.

This might be related to or satisfied by the "tahoe-deps" and/or SUMO tarball features, which we have sort-of working right now.

Let's use this ticket to work out what test would satisfy this feature request. gdt: how could we test this? Maybe just do a test like the one described in comment:21:ticket:1220 (the one which will go red if the code under test says "Downloading http") but we first run python setup.py justdeps. So then after running python setup.py justdeps, then it runs python setup.py justbuild and goes red if it says "Downloading http", then it runs python setup.py justinstall --prefix=$PREFIXDIR (and again goes red if it says "Downloading http"), then it runs $PREFIXDIR/bin/tahoe --version-and-path and goes red unless it prints out the right version and path.

Note that the test code doesn't have a fair chance of marking the code under test red if the system already has all deps already installed. There have to be some missing deps so that the code under test might be tempted to try to download those deps when it isn't supposed to.