In the dependent change, the docker roles will add sibling packages to
the .zuul-siblings directory of the checked-out source.
Refactor the "assemble" script to handle this. Essentially we build
the wheel for "." and then iterate over ZUUL_SIBLINGS subdirectories
(set in a --build-arg by the role in dependent change) to also build
the sibling packages. Note we concatenate the bindep.txt files, so
that we end up with the complete package list required by the main
code and its dependencies.
"install-from-bindep" now installs all the wheels, using --force to
make sure we re-install the speculatively built packages.
This means that a single Dockerfile works under Zuul when
ZUUL_SIBLINGS is set, pointing to Zuul's checkouts; but it also works
stand-alone -- in this case ZUUL_SIBLINGS is empty and we just install
from upstream as usual.
Depends-On: https://review.opendev.org/696987
Change-Id: I4943ae723b06b0ad808e7c7f20788109e21aa8bf
bd66a7cb1b