[array api] fix deprecation to support old import pattern

[jakevdp]

Followup to #22818

The previous approach made it so that a common import pattern didn't work:

>>> import jax.experimental.array_api as nx
ModuleNotFoundError: No module named 'jax.experimental.array_api'

We need to keep a stub submodule here, but patch sys.modules to return the new path. With this change, it looks like this:

In [1]: import jax.experimental.array_api as nx
<ipython-input-1-9d2abc00e9fa>:1: DeprecationWarning: jax.experimental.array_api import is no longer required as of JAX v0.4.32; jax.numpy supports the array API by default.
  import jax.experimental.array_api as nx

In [2]: import jax.numpy as jnp

In [3]: nx is jnp
Out[3]: True

[jakevdp]

I also tested this locally with both the released version of array_api_compat and with data-apis/array-api-compat#172, and it works correctly in both cases.