A comprehensive refactor of build-dep and related pkgstyle functionality
to install or list (-l/--list) build deps from templates as well as
from source packages.

The new default approach of parsing build deps from templates has
following advantages:

* works directly on project source without archive/srcpkg build
* requires less tools to work
* is faster

Alternatively, -s/--srcpkg makes apkg parse build deps from source
package which is slower but more robust in some cases.

Build deps can also be extracted from archive containing proper distro/
files with -a/--archive option in both template and --srcpkg mode.

-u/--upstream option uses upstream archive (get-archive) to get build
deps for maximum convenience.

Fixes: #34