This commit reworks the bootstrapping procedure to use Spack environments
as much as possible.
The `spack.bootstrap` module has also been reorganized into a Python package.
A distinction is made among "core" Spack dependencies (clingo, GnuPG, patchelf)
and other dependencies. For a number of reasons, explained in the `spack.bootstrap.core`
module docstring, "core" dependencies are bootstrapped with the current ad-hoc
method.
All the other dependencies are instead bootstrapped using a Spack environment
that lives in a directory specific to the interpreter and the architecture being used.
4c05fe569c