Add 'make srcdist' target

[gavanderhoorn]

Prepares a directory that contains:

• sources of all pkgs in the source space, and those of their deps
• include directories exported by the pkgs in the source space, and those of their deps
• a readme/index file with information on the versions of the pkgs from which the sources came

Optionally, it could create a zip/tarball/x of that directory.

Such a 'source distribution archive' would make distributing (open-source) applications more convenient, as it would make building all sources on a system without rossum and ktransw easier.