EMSUSD-623 edit as Maya multiple variants

[pierrebai-adsk]

This change adds support to edit-as-Maya a prim that is under a variant, with
each variation having a different Maya reference. That is, a single prim can
now be edited-as-Maya multiple times in parallel. This required major changes
on how those edited prim are tracked.

Edit-as-Maya authors data in the session layer related to the edited prim.
Previously, this data was authored outside any variant, which caused problems
when a prim had multiple variants. Now these data are authored inside the same
variant as the prim itself.

Additional helper functions:

• Add the getEditTargetForVariants function helper function to create the USD edit target targeting all the variants that affect a given prim.
• Add variant-specific pull info metadata in the pull-info helper functions.

Change to the orphan manager:

• Use the variant-targeting helper function when authoring the Maya reference custom attribute that holds the name of the Maya reference node.
• Support having multiple entries for a given UFE path.
• To differentiate between entries, the corresponding root Maya DAG path must be given in all functions to figure out which version of the prim we are dealing with.
• Fortunately, all callers always have on hand the DAG path of the root Maya node.
• Change all implementation function to iterate over these variations.
• Change the orphaned nodes serialization to support the extra data.
• Make the serialization format backward compatible by special-casing the usual case of having a single variation at a given edited UFE path.
• Add a log to tell the user when an edited prim is orphaned.

Changes to the prim updater manager:

• Ignore edited prim that are orphaned when determining if a prim has edited descendants.
• Adapt to changes to the orphaned manager by passing the Maya DAG path to the root of the edited nodes.

Changes to Maya reference translator (importer from USD to Maya):

• Use the variant-targeting helper function when authoring the pulled-prim metadata.
• Make many functions be purely in the implementation instead of declared private in the class.
• This allows better hiding of the implementation.
• This was also necessary to make the function callable from internal implementation functions.
• Renamed LoadMayaReference to CreateMayaReference to better reflect what the function does.
• Split the long update function into multiple function for clarity.
• When trying to reuse Maya reference nodes, verify that the referenced file is the one that was expected.
• This allows two prim with the same USD path but different references to work alongside each other.
• This happens when one prim with two variants each contain a Maya reference.
• Add more comments in the code to explain what is going on.
• Add a log to tell the user when a reference does not have the expected file path.