Replace PUBLIC_{HEADERS,CLASSES} with PRIVATE for plugins

[nvmkuruc]

Description of Change(s)

While investigating the feasibility of using the MODULE cmake library type to facilitate exporting targets for plugins, I observed that several plugins provided PUBLIC_HEADERS and PUBLIC_CLASSES. (This was previously noted in #4079 as well).

Usage of pxr_plugin is for shared libraries that aren't intended to be linked externally linked against. It's not immediately obvious that these headers are useable outside of the pxr project and that their being tagged as PUBLIC_ wasn't an accident.

This change removes these headers from the installed include directory.

As an aside, I could imagine that for pure organizational purposes, there being a case for pure public headers without any linkage requirements, but that doesn't appear what these are. Future changes might consider disallowing the PUBLIC_ arguments from pxr_plugin. Given that pxr_library offers "libraries with plugin semantics" (libraries with resources and plugInfo.json), there doesn't seem to be a reason to also offer "plugins with semi-library semantics".

Link to proposal (if applicable)

Fixes Issue(s)

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:
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[jesschimein]

Filed as internal issue #USD-12602

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