Building now works! \o/ (but assembling still doesn’t)

	- Test module removed, as it didn’t make any real test. Or at
	  least not as much as the stuff it screwed.
	- It’s still not very useful, considering that we don’t have
	  automated variable substitutions, but at least it does
	  somewhat work.
Failed downloads do not leave an empty file behind anymore.
Some reorganization work.

	- The plan is to, at some point, have a libmpkgmk on which
	  modules (or other, external tools) can link to avoid having to
	  copy stuff from mpkgmk.
	- The headers are presently not installed.
	- mpkgmk_private might disappear in the future. I’m just not
	  sure I want the prefixed variables everywhere. :(
libmpkgmk \o/

	- Currently, it only contains the three UI functions (info,
	  warning and error) and the shell() function used in build().
	- Both mpkgmk.h and mpkgmk_private.h are installed. For now.
	  mpkgmk_private.h is installed in case third-party modules
	  might want to use it.
	- Our build.zsh got upgrades for both libraries and modules.
	  Statically linked libraries are still unsupported, though.
	  Making symlinks at installation time for libraries is
	  unsupported as well.