Add spack --bootstrap option for accessing bootstrap store (#25601)

We can see what is in the bootstrap store with `spack find -b`, and you can clean it with `spack
clean -b`, but we can't do much else with it, and if there are bootstrap issues they can be hard to
debug.

We already have `spack --mock`, which allows you to swap in the mock packages from the command
line. This PR introduces `spack -b` / `spack --bootstrap`, which runs all of spack with
`ensure_bootstrap_configuration()` set. This means that you can run `spack -b find`, `spack -b
install`, `spack -b spec`, etc. to see what *would* happen with bootstrap configuration, to remove
specific bootstrap packages, etc. This will hopefully make developers' lives easier as they deal
with bootstrap packages.

This PR also uses a `nullcontext` context manager. `nullcontext` has been implemented in several
other places in Spack, and this PR consolidates them to `llnl.util.lang`, with a note that we can
delete the function if we ever reqyire a new enough Python.

- [x] introduce `spack --bootstrap` option
- [x] consolidated all `nullcontext` usages to `llnl.util.lang`

lib/spack/llnl/util/lang.py

@@ -5,6 +5,7 @@
 
 from __future__ import division
 
+import contextlib
 import functools
 import inspect
 import os
@@ -921,3 +922,11 @@ def elide_list(line_list, max_num=10):
         return line_list[:max_num - 1] + ['...'] + line_list[-1:]
     else:
         return line_list
+
+
+@contextlib.contextmanager
+def nullcontext(*args, **kwargs):
+    """Empty context manager.
+    TODO: replace with contextlib.nullcontext() if we ever require python 3.7.
+    """
+    yield