Tab completion slow

Problem

Tab completion is getting really slow on projects of decent size. I was able to track it down to the _just_load_justfile call. This is where everything is loaded so that plugins can be discovered, and parsed. The timing for this call can be emulated via:

$ time bash -ic ". setup.env; . external/vsi_common/linux/source_once.bsh; _just_load_justfile Justfile"

real    0m4.014s
user    0m2.723s
sys     0m1.296s

This is because, source_once is disabled (by design) in interactive mode:

$ time bash -c ". setup.env; . external/vsi_common/linux/source_once.bsh; _just_load_justfile Justfile"

real    0m0.316s
user    0m0.220s
sys     0m0.105s

Possible Solutions:

1. Add an additional flag, so that tab complete can run in a subshell, and go ahead and use source_once

if [[ $- != *i* -o ${JUST_SOURCE_ONCE} = 0 ]]; then
  source_once &> /dev/null && return 0
fi

2. Come up with a new way to discover plugins. Forcing the use of JUST_PLUGIN_FILE could work here, but would involve multiple steps of refactoring:
   1. Add a check into all plugins, that error out if they aren't being sourced from _just_get_plugins (a simple check gate for a private variable would be good enough)
   2. All projects would need to be updated to use this
   3. Dev would have to get used to this

Option 2 does have the advantage that potentially even faster tab completion could be accomplished, eventually on windows too.

Ideas? @scott-vsi @drewgilliam

Start by reproducing the timing with the provided bash command, then inspect _just_load_justfile, external/vsi_common/linux/source_once.bsh, and the _just_get_plugins/JUST_PLUGIN_FILE paths mentioned in the issue. Compare the proposed approaches and define done as faster tab completion while preserving plugin discovery and existing interactive behavior.