Patch usddiff for USDZ containing crate layers

[andrewkaufman]

Description of Change(s)

The underlying layers need to be converted to usda before they are diffed.

Previously a crate layer within a USDZ would produce the following failure when run through usddiff:

Differences in a.usdz[basic.usdc] and b.usdz[basic.usdc]:
'utf-8' codec can't decode byte 0xfe in position 16: invalid start byte

Note that there are 2 test changes here:

• A new test using the same composed stage as the original testDiffToolIterateUsdz, but with the internal basic.usda converted to basic.usdc.
  • This was done by unzipping the original a.usdz and b.usdz, using usdcat to convert the layer, then usdzip to make the new archives.
• A change to the expected output in testDiffToolIterateUsdz
  • This is due to all layers within a USDZ running through usdcat now, which did not occur previously.
  • This is an additional bug fix, as _runDiff is called unconditionally on non-zipped layers, so should also be called unconditionally on internal layers. Previously, whitespace diffs in USDA layers would cause failures when embedded in USDZ, but would not cause failures if extracted first & diff'd outside the USDZ. Now the diff result is consistent whether the usda is tested on its own or within an archive.

Checklist

• I have created this PR based on the dev branch
• I have followed the coding conventions
• I have added unit tests that exercise this functionality (Reference:
  testing guidelines)
• I have verified that all unit tests pass with the proposed changes
• I have submitted a signed Contributor License Agreement (Reference:
  Contributor License Agreement instructions)

[jesschimein]

Filed as internal issue #USD-10468

[jesschimein]

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