primvars on packed agents

**Describe the bug**
Im very sorry in advance, this will probably by the worst bug report ever.
I have zero knowledge of USD, yet i have wound some bug that doesnt exist in Karma and its connected to primvars. If i have primvars on top of packed agents, those primvars are not visible to arnold. Only the primvars inside the packed agents are visible.

**To Reproduce**
1 Create 4 crowd agents, where primvar TEST is inside each agent and random valued TEST2 is on packed agents.

2 Import to Solaris with sopimport node and enable sourcing all attributes. In my sceneGraphDetails i will see both TEST and TEST2 primvars with values.

3 With Arnold i can get values from TEST, they will all be the same as values i can see when unpacking agents at SOP level. But if i try to access values from TEST2, primvars that are being added on top of packed agents, i will not get any values

**Expected behavior**
In Karma, and thats what i would expect, i can get values TEST2 and use them at shader lever

**Used Software Versions**
- Arnold: [6.0.3.0beta14]

Reproduce the report by creating four crowd agents, placing TEST inside each agent and random TEST2 on the packed agents, then importing through the SOP Import node with all attributes sourced. Compare sceneGraphDetails and Arnold shader access with Karma; done means Arnold exposes TEST2 values at shader level as described.

Written by the indexing model from the issue text.