`spack --debug config edit` was not working properly -- it would not do show a
stack trace for configuration errors.
- [x] Rework `_main()` and add some notes for maintainers on where things need
to go for configuration to work properly.
- [x] Move config setup to *after* command-line parsing is done.
Co-authored-by: scheibelp <scheibel1@llnl.gov>