Bootstrap most of Spack dependencies using environments (#34029)

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.

share/spack/templates/bootstrap/spack.yaml

@@ -0,0 +1,34 @@
+# This environment contains Spack non-core dependencies for the
+# following configuration
+#
+# Python spec: {{ python_spec }}
+# Python interpreter: {{ python_prefix }}
+# Architecture: {{ architecture }}
+#
+spack:
+  specs:
+{% for spec in environment_specs %}
+    - "{{ spec }}"
+{% endfor %}
+  view: {{ environment_path }}/view
+
+  config:
+    install_tree:
+      root: {{ store_path }}
+
+  packages:
+    python:
+      buildable: false
+      externals:
+        - spec: "{{ python_spec }}"
+          prefix: "{{ python_prefix }}"
+
+    py-typed-ast:
+      require: "+wheel"
+
+    py-platformdirs:
+      require: "+wheel"
+
+  concretizer:
+    reuse: false
+    unify: true