[bazel] Create jax/_src/BUILD and move _src build declarations
This ensures that nobody inadvertently adds new `_src` files to top-level build rules. Keeping _src builds separate means that _src imports are subject to visibility restrictions under strict dependency management.
This change keeps the old build targets as aliases for now; the only exception is the `_src/export/serialization.fbs` file export, which is an invalid target name once `_src` becomes a standalone package. I'll remove the aliases in a followup after migrating downstream build rules to the new targets.
PiperOrigin-RevId: 786710168
