Remove assumption in UsdUtils_FileAnalyzer that USDZ files are fully self contained

[dgovil]

Description of Change(s)

UsdUtils_FileAnalyzer assumed that USDZ files are fully self contained.
I'm removing this assumption because:

1. According to prior discussion, USDZ files may perhaps refer to external files, even if that's not ideal
2. This was skipping validation in files that did have missing references, thereby not catching incorrect files

Fixes Issue(s)

2636

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

• I have submitted a signed Contributor License Agreement

[meshula]

Irrespective of point 1,

USDZ files may perhaps refer to external files

the test you removed is making a pretty big assumption that the referenced package itself is valid. USDZ files do have the requirement that they are reproducibly resolvable, so removing the "early out" from the test should be both safe to do, and increase confidence about the overall analysis. My follow up question would be whether the analyzer checks for the "anchored" restriction, or if there might be further work required?

For Reproducible Results, Encapsulate Using Anchored Asset Paths

To insure uniform consumption of assets shipped to clients via usdz, we feel it
is prudent to curtail the power of USD's asset resolution system, so that the
asset references within a package resolve uniformly on any consuming system,
regardless of how that system's USD ArResolver is configured. Rather than make
restrictions within the FileFormat itself, we propose that such "encapsulation"
of packages be achieved by restricting the content to only use anchored
paths (paths that begin with "./" or "../", both of which are interpreted in the
virtual filesystem described by the package's internal layout.

[dgovil]

I'm not sure on that front. I'd have to go code spelunking, but at a cursory glance I don't believe it does.
However there is ARKitPackageEncapsulationChecker at the UseChecker level that does check for encapsulation, but only for assets that actually resolve which was why nothing was flagging it.

[jesschimein]

Filed as internal issue #USD-8641

[meshula]

Reopened, as the noted commit only partially addressed the PR.

[dgovil]

@meshula It looks like this was fixed somewhere along the way in the last few USD versions, even though I can't find a commit that intentionally did so. I'll (re)close this one out.