refactor import visit strategy, use separate visit_launcher module (try 2)

[cyrush]

Description

New branch and PR that supersedes: #20487

Resolves #18039 and #18884

Since Python3 we have had challenges with importing visit into a stand alone python interpreter due to module scoping and name spacing. In the past we provided an all in one module with a small front end that located a selected visit install, and then from there did a dynamic import. The delayed import caused issues with things like visit_utils, where were imported before visit was fully ready.

To resolve this, and avoid things like

import visit
visit.Launch()
import visit

The frontend logic is now in a separate module named visit_launcher

import visit_launcher
visit_launcher.Launch()
import visit

We will lose some functionally - visit_launcher.Launch() may only be able to launch for 3.5.0 or newer installs.

I still have a few things to iron out, but with the current implementation we can run the following:

import visit_launcher
visit_launcher.Launch(vdir="/Users/harrison37/Work/github/visit-dav/visit/build_dev/")

import visit
print(dir(visit))
from visit import *

import visit_utils
visit_utils.engine.open(vdir="/Users/harrison37/Work/github/visit-dav/visit/build_dev/")
OpenDatabase("/Users/harrison37/Desktop/visit_tutorial/current/data/rect3d.silo")
AddPlot("Pseudocolor","d")
DrawPlots()
SaveWindow()

Which we could not run before .

Type of change

• Bug fix~~
• New feature~~
• Documentation update~~
• Other~~

How Has This Been Tested?

Tested on macOS

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Checklist:

• I have commented my code where applicable.~~
• I have updated the release notes.~~
• I have made corresponding changes to the documentation.~~
  - [ ] I have added debugging support to my changes.
  - [ ] I have added tests that prove my fix is effective or that my feature works.
  - [ ] I have confirmed new and existing unit tests pass locally with my changes.
  - [ ] I have added new baselines for any new tests to the repo.
  - [ ] I have NOT made any changes to protocol or public interfaces in an RC branch.