build: add new command to build packages

Newly introduced

    apkg build

command is able to build packages from source packages
and use `apkg srcpkg` to create them if needed.

Following pkstyles support build_packages():

* arch: using makepkg
* deb: using dpkg-buildpackage

pkgstyle.deb.build_package() also contains preliminary support of
isolated build using pbuilder which isn't enabled yet but it I tested it
successfully.

This patch also includes some polish and refactoring all over the place
as archive -> srcpkg -> build commands are connected.

Simple self test with apkg calling `apkg build` on itself is included.