[Upgrade] Fix 16952: Fix ImportError after upgrade

[fengzhou-msft]

Description

Resolve #16952

The issue was caused by reloading the azure.cli.core python module to get the new version after upgrade, it depends on modules in knack. The new loaded azure.cli.core depends on new code of a new knack version: from knack.util import status_tag_messages, but knack.util is not reloaded, causing the issue of ImportError.

There are two ways to solve this issue:

1. Reload all loaded modules in sys.modules starting with knack such as knack.util, knack.log. Note that knack and knack.util are 2 items in sys.modules, simply reloading knack will not fix the ImportError with from knack.util import status_tag_messages.
2. Use subprocess to get az version to avoid calling cached modules in the same Python Interpreter.

This PR uses the second method as it is more error-proofing and easier to maintain as azure.cli.core could rely on new modules in the future.

Testing Guide

1. Create a clean virtual env and activate it
   python -m venv env-test
2. Install the old version of azure-cli
pip install azure-cli==2.17.1
3. Modify this file to apply changes in this PR:
   env-test/lib/site-packages/azure/cli/command_modules/util/custom.py
4. Run az upgrade

History Notes

[Component Name 1] BREAKING CHANGE: az command a: Make some customer-facing breaking change.
[Component Name 2] az command b: Add some customer-facing feature.

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[jiasli]

Elegant solution!