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).