[openusd_internal] Patch pegtl namespace calculation

[rpoyner-tri]

This patch allows the drake_vendor namespace modifications to be reflected in TAO_PEGTL_NAMESPACE, which in turn allows the resulting symbols to be matched by allow lists in exported_symbols_test.

This is yet another installment, and not the last, in the openusd_internal symbol-hiding saga.

---

This change is 

[rpoyner-tri]

Closing this one, to combine it with other symbol leak fixes.

[jwnimmer-tri]

My preference would actually be to just push the symbol patching into Hong's the actual parsing PR. When we try to land these ahead of time, they are at least partway dead code (no CI coverage). If the parsing PR is too big, we can carve it into like spheres-only vs the rest or something.