Material-X: flatten filenames upon read #974

[Derek-Nvidia]

Description of Change(s)

1. Call mx::flattenFilenames() anytime a mx::read* function is called. This results in a document in which all of the 'fileprefix' attributes have been fully resolved and prepended to any inputs of type 'filename'
2. Add test_ExpandFilePrefix to TestFileFormat to test fileprefix resolution at various scopes in the MaterialX document.

Fixes Issue(s)

• usdMtlx ignores an active file prefix #974

• I have verified that all unit tests pass with the proposed changes

• I have submitted a signed Contributor License Agreement

[jesschimein]

Filed as internal issue #USD-8666