GeezerVison

Each extension statically links protobuf, abseil and cel-cpp. On macOS,
pybind_extension links with only `-undefined dynamic_lookup`, so those
copies are exported, including thousands of weak definitions. dyld
coalesces exported weak definitions across every loaded image, so
importing cel_expr_python alongside another extension that ships protobuf
(e.g. ortools) binds one runtime's code to the other's parse tables and
crashes in AddDescriptors at import time.

Wrap pybind_extension to add `-Wl,-exported_symbol,_PyInit_<name>` on
macOS, matching the isolation Linux already gets from `-Wl,-Bsymbolic`,
and add a macOS test that each extension exports only its init function.

Fixes cel-expr#115

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