implement upstream workflow

apkg srcpkg/build now use distro/ input files from tarball when -u/--upstream is specified.

This allows building of (source) packages from an archive containing distro/ dir without the need for any local project files:

apkg build --upstream --archive banana-1.2.3.tar.gz

Once upstream tarballs include apkg distro/ files, building latest upstream package from within a project repo should be as easy as:

apkg build -u

Fixes: #14 (closed)