[export] Refactor the imports for the public API of jax.experimental.export

[copybara-service]

[export] Refactor the imports for the public API of jax.experimental.export

Previously we used from jax.experimental.export import export and
export.export(fun). Now we want to add the public API directly
to jax.experimental.export, for the following desired usage:

from jax.experimental import export

exp: export.Exported = export.export(fun)
ser: bytearray = export.serialize(exp)
exp1: export.Exported = export.deserialized(ser)
export.call(exp1)

This change requires changing the type of
jax.experimental.export.export from a
module to a function. This confuses
pytype for the targets with strict type checking,
which is why I attempt to make this change
atomically throughout the internal code base.

In order to support backwards compatibility with
OSS packages, this change also includes explicit
JAX version checks in several OSS packages, and
also adds to the export function the attributes
that the old export module had.