Cherry-pick of #16388 to RC3.6.0_master

Purpose

When Dynamo is closed and reopened in D4R, the AvailableEngines collection under PythonEngineManager is not changed so the PythonEngineAdded is never triggered when Dynamo is reopened. This has caused this bug https://jira.autodesk.com/browse/DYN-9082.

So now, when Dynamo is closed, we clear the AvailableEngines collection and add the default python engine back when it s started back. The rest of the python engines, if any, will be loaded from the packages extension.

Declarations

Check these if you believe they are true

• Is documented according to the standards
• The level of testing this PR includes is appropriate
• User facing strings, if any, are extracted into *.resx files
• Snapshot of UI changes, if any.
• Changes to the API follow Semantic Versioning and are documented in the API Changes document.
• This PR modifies some build requirements and the readme is updated
• This PR contains no files larger than 50 MB
• This PR introduces new feature code involve network connecting and is tested with no-network mode.

Release Notes

[DYN-9082] All Python Engines should be loaded correctly in D4R

Reviewers

@DynamoDS/eidos