Automatic loading of ExportChaser and ImportChaser plugins

[jufrantz]

This PR adds ability for automatic loading for USD or Maya plugins that register an ExportChaser or ImportChaser.

With this change, clients no longer need to preload chaser plugins manually, wrap the MayaUSD import/export commands, load plugins at Maya startup, or rely on a JobContext plugins side effect to make a chaser available.

The implementation follows the existing JobContextPlugin and ShadingModePlugin discovery pattern. Plugins can advertise chaser support in plugInfo.json with:

"UsdMaya": {
  "ExportChaserPlugin": {
    "mayaPlugin": "myExportChaserPlugin"
  }
}

or:

"UsdMaya": {
  "ImportChaserPlugin": {
    "mayaPlugin": "myImportChaserPlugin"
  }
}

If mayaPlugin is omitted, the USD plugin is loaded directly.

Included changes:

• Refactors the shared plugin discovery/loading logic used by jobContext and shadingMode registries
• Reuses that logic for import and export chasers, triggering chaser plugin loading just before chasers are created.
• Adds unit tests for automatic import and export chaser loading.
• Updates related documumentation.

[seando-adsk]

@jufrantz We discussed this PR internally and one comment that came up was if "this PR looking to add the option to make it automatic or not or will it force to automatically load?" Looking quickly at the changes I don't see any option - it is just automatic. Is that correct? If so it was suggested this could be a problem and changing behaviours for other customers then. And could there be an option then?

Sean

[jufrantz]

Hi @seando-adsk,

There is no global option, but this PR should not change the behavior of existing plugins. To opt in to this automatic loading, the developer needs to add new metadata (Info/UsdMaya/ExportChaserPlugin) to the plugInfo.json manifest file, as in this unit test file: infoImportChaser_plugInfo.json.

Therefore, existing plugins, that do not include the new metadata, will not be loaded automatically.

Julien

[seando-adsk]

@jufrantz Thanks for the answer. That covers all the concerns we had. I'll get someone assigned for a code review.

Sean