pip-install-swh-dev: install packages all at once
With recent enough versions of setuptools/setuptools-scm, the pip dependency resolver is now capable of sorting out our full dependency tree. We can just install all packages at once (which does the dependency resolution step once).
To speed up the process even more, inline the few "build" (setup.py) dependencies to allow disabling build isolation (which creates a new venv and installs setuptools-scm in it for every single package).
Test Plan
tox -r -e sphinx-dev --notest
shows all depepdencies come from files in ../
Migrated from D7736 (view on Phabricator)