Previous code was rebuilding "name to material" map for each object being imported. Which means O(N*M) complexity (N=object count, M=material count). There was already a TODO comment suggesting that a single map that's maintained for the whole import would be enough. This commit does exactly that.

While importing Moana USD scene (260k objects, 18k materials) this saves about 6 minutes of import time. Without other outstanding patches this is not "massive", but if all of (D14162, D15145, D15215) are applied then it gets import time from 16 minutes down to 10 minutes, which is not too bad.