Very basic weld support.
Add Choice and other stuff

The new Choice class is intended to allow choosing one of several
options depending on a target string.

In support, add a mechanism for extracting information from
/etc/os-release (which Python's platform.py doesn't yet look at),
and thus get_os_version_name(), to do more-or-less what it says.
Add the ability to aptget based on OS version

This uses the new Choices mechanism to allow choosing a
package on the basis of the available OS.

It's possible to specify the OS version name just in case we
want to do this for cross compilation (of an Ubuntu system)
Some basic testing of the Choices and aptget changes

Note that for aptget in particular it really only checks that
build descriptions are valid or not - it doesn't actually check
that the aptget call produces the correct internal result.