Add git repository aliasing and quick module declaration to kdesrc-build.

This commit adds two options (not yet documented in the DocBook):

1. git-repository-base, which defines a new repository alias (kde-git in the sample file)

In order to actually use this repository alias, you can use a new type of module
(unimagineatively called a "module-set") like so:

global
...
end global

module-set $repo_name
use-modules <list of git modules e.g. akonadi attica soprano k3b etc.>
end module-set

No other options are supported in a module-set at this point. kdesrc-build treats a
module-set as if you'd simply manually declared a new module for each item you passed
to use-modules, and sets the "repository" option for each new module to the base URL
(as determined by repo_name), with the module name tacked on.

kdesrc-build will filter any trailing .git off of the virtual module name.

If you still want to add options for a module, you can simply manually declare a module
of that name after the module-set, and choose options there (it won't change the build
order though).

Hopefully this makes it that much easier to use the plethora of git modules that have
been getting successfully transferred over to git.kde.org. PLEASE let me know if I've
broken something here or otherwise improperly implemented it. CC'ing dfaure since I
figure he'll be interested and/or have feedback.