python compiled extensions should never need to expose any symbol other
than PyInit_* which is declared with default visibility via
PyMODINIT_FUNC on supported compilers.
Thus, a reasonably sane default is to mark any other symbols as hidden,
while still respecting any manually specified visibility.
Gate this on the version of python itself, as not all versions decorate
PyMODINIT_FUNC properly.
This mirrors the modules keyword argument that some dependencies (such
as qt and llvm) take. This allows an easier method to determine if
modules are installed.
When dependency(), find_library(), find_program(), or
python.find_installation() return a not-found object and disabler is
true, they return a Disabler object instead.
Eli Schwartz