Apple: Add ability to use Xcode debugger with OpenUSD

[slingthor]

Description of Change(s)

Exposes "PXR_SHARED_LIBRARY_LOCATION" to add the ability for Xcode debugger to debug OpenUSD.
N/A to unit test this

Fixes Issue(s)

Inability to debug OpenUSD with Xcode

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-10904

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[jesschimein]

/AzurePipelines run

[davidgyu]

Hi Thor!

You should be able to accomplish this by setting the following environment vars in your Xcode scheme:

PYTHONPATH=/path/to/my_usd_install_dir/lib/python
PXR_PLUGINPATH_NAME=/path/to/my_usd_install_dir/lib/usd:/path/to/my_usd_install_dir/plugin/usd

Note that the second env var has two paths separated by a colon, and plugin discovery will search both paths.

[tallytalwar]

/AzurePipelines run

[tallytalwar]

/AzurePipelines run

[jesschimein]

/AzurePipelines run