Regression in commit a3d287c553.
When a given kwarg is not specified, we want to not generate it as a
simd variant. Since the default for buildtarget sources is `[]` it
resulted in building a static library with no sources, and a warning
stating that this was buggy and will eventually be removed.
Fix this by teaching buildtarget sources to allow None, and defaulting
to it specifically for the simd module. We can check this and then skip
processing entirely.
Fixes#12438
Thanks to `ModuleInfo`, all modules are just named `foo.py` instead of
`unstable_foo.py`, which simplifies the import method a bit. This also
allows for accurate FeatureNew/FeatureDeprecated use, as we know when
the module was added and if/when it was stabilized.