RFE: srcpkgs from --upstream archive templates

By default apkg srcpkg and apkg build operate in dev mode where packaging templates distro/pkg/*/ are used to create source package along with specified archive.

In --upstream mode, srcpkg should use packaging templates from the specified archive instead.

• unpack archive somewhere, perhaps pkg/archives/unpacked/project-1.2.3
• use pkg/archives/unpacked/project-1.2.3/distro/* to build source package as opposed to repo's ./distro/*
• build is unchanged by this as it operates on srcpkgs

Consider splitting pkg/srcpkg into pkg/srcpkg/dev and pkg/srcpkg/upstream and pkg equivalent to avoid collisions between dev and upstream files.