[USD] Installing built-in schemas.

[sirpalee]

Description of Change(s)

The following changes are included:

• Installing the schemas next to generatedSchema.usda.
• Massaging the installed schema to work well outside the usd source-tree.
• Adding a new function to PlugRegistry (and its python binding) that allows users to query all the search paths for plugins.
• Modifying usdGenSchema to pass these paths to the default resolver, making the external plugins easier to setup.

Fixes Issue(s)

#158

[jtran56]

Filed as internal issue #148279.

[sunyab]

Thank you for this PR! It was extremely useful in proving out the search path-based direction for solving this issue. We've just landed several changes to address #158 that we think should take care of this without requiring build-time modifications of the installed schema.usda files.

I'm going to close this PR out in favor of those changes. Thanks again!