makechrootpkg: Avoid parsing PKGBUILD and support VCS sources

 - Ensure sources are available before entering chroot
 - Bind STARTDIR and SRCDEST into the chroot read-only
 - Refactor makechrootpkg and introduce meaningful functions

Avoids copying stuff from/to the chroot as much as possible. With
VCS sources these copies can get quite expensive.
arch-nspawn: setarch to CARCH

Allows calling makechrootpkg without worrying about the architecture