* Add add_halide_python_extension_library() rule

This adds a rule to create a single Python extension library from one (or more) halide_library rules. This allows you to package multiple Halide filters into a single Python module, which is nice because (1) being able to organize is good, and (2) all the filters in a single Python extension module share the same Halide runtime, including (e.g.) thread pools and method overrides.

(It also removes the just-recently-added PYTHON_EXTENSION_LIBRARY option from the add_halide_library rule, as this new rule is better and more flexible in pretty much every way.)

This modifies the content of our `python_extension` output in such a way that existing uses should be completely unaffected, but defining the right preprocessor macros allows us to split the function wrappers up from the method-definition declaration, so we don't have to generate any new code artifiacts to make this work.

Partially addresses #6956.

* Omits -D in target_compile_definitions

* be explicit about setting to empty

* Add quotes

* Add comments re BUILD_INTERFACE

* Add MODULE_NAME comment

* Remove "defined in HalideGeneratorHelpers.cmake"

* Add comment re add_halide_runtime()

* osx, macos, darwin, oh m

* blankity blank blank

* Use OBJECT library instead

* Add comment about X-macros

* Update HalideGeneratorHelpers.cmake