Install packages from PIP_PACKAGES before wheel build to be able
to limit dependencies specified in "setup_requires" of packages
listed in upper-constraints.txt, that pip has no way to control.
This is intended to work around https://github.com/lxc/pylxd/issues/308
or similar issues.
Change-Id: I36bd4347727b4c7f4beea2cdba6674445fbb4ca4