consider bootstrapping spack as a standalone PEX to avoid interpreter clashes

[cosmicexplorer]

Summary: Expose the pex python tool as a spack package. This allows creating hermetic python environments as executable zip files, otherwise known as PEX files.

Rationale

• Even after bootstrapping: bootstrap spack dependencies (executable and python module) #20207, the version of python used to bootstrap clingo must remain the first python executable on the PATH -- otherwise another python executable is used to execute spack, which will then fail to import clingo.
  • If the user wants to use a different python version, they then lose the ability to concretize with clingo.
• If spack's source files and bootstrapped python dependencies from bootstrapping: bootstrap spack dependencies (executable and python module) #20207 are made into a PEX file, the correct interpreter will be selected each time, just for that PEX process, and we can remove the shell loop at the top of bin/spack!
• It would also allow bootstrapping spack on a remote machine without git installed, by cping the PEX file

Description

1. Add a PexPackage class which exposes the pex package.
   • It should probably download it from github releases, as that avoids any dependency on a specific python version at install time.
2. Create a PEX file for spack itself after bootstrapping with bootstrapping: bootstrap spack dependencies (executable and python module) #20207.
   • This should not modify the bootstrap process from that PR whatsoever.
3. Fix the spack PEX so spack executes from its checkout directory, not in ~/.pex.
   • The spack PEX will use the unpack directory within ~/.pex as its checkout directory by default, since that is where pex puts the actual files.
   • Can "hardcode" a __main__ script which makes spack execute in the correct checkout directory.
     • This should be doable without editing bin/spack at all, but that could also work.

[cosmicexplorer]

This may not end up being needed. But I would prefer we avoid interpreter entanglement rather than letting some users experience issues with isolation after bootstrapping clingo, and learn to accept that that's necessary.