make SourceRepo.parse_pkgbuild static

It'll be used outside of a SourceRepo instance soon, to verify the
version of retrieved package source.

Signed-off-by: Paul Burton <paul@archlinuxmips.org>
add "build ready" command

Add a command which identifies any packaged pending a build whose
dependencies are all met & up to date.

Signed-off-by: Paul Burton <paul@archlinuxmips.org>
add "build source" command

Find the source for a package. Currently only handles source retrieved
from upstream package repositories.

Signed-off-by: Paul Burton <paul@archlinuxmips.org>
allow build ready to produce json output