Add support for projects.kde.org XML database.

This commit adds initial support for the projects.kde.org XML database as exported by https://projects.kde.org/kde_projects.xml

The short story is, to use the "use-xml-modules" option in a *module-set*. E.g., what I just tested was with

module-set
use-xml-modules phonon-gstreamer
end module-set

If you want --pretend to work you should throw a module set that uses
XML modules in somewhere and try having the XML-based module built, or
just download kde_projects.xml yourself and plop it right in the base
source directory.

In theory a grouping module (such as extragear/utils) should allow you
to recursively grab and build every module composed within it.

However, modules that contain a <repo> tag, and then have children
underneath also containing <repo> tags only have the very base <repo>
tag working (if I coded it right ;)

The reasoning for this is to allow module groups where you can build the
entire module, or given subsets of it (e.g. Calligra allows this from
what I can tell, you can build Calligra or assuming you've met the
dependencies, Krita).

If a module has no direct <repo> tag, but includes modules or projects
or whatever that each have their own <repo>, then everything should be
recursively built.

kdesrc-build obeys the <active> tag, so modules that claim not to be
active will not be built by kdesrc-build (at this point, even if
directly passed on the command line, although that may change later).

I'm super-tired and won't have time to test throughout much of this
week, so I've put it in a branch. Feel free to test yourself if you're
interested though!

I'd like to thank Allen Winter who contributed the core XML parsing
code, it was a big boost to getting something working. :)