switching to _src structure

@jheek mentioned: "this refactor [#2807]...was rolled back because somebody imports the ones initializer from nn.linen.attention. I have seen more of these odd dependencies on Flax lately. I think we should prioritize switching to a _src structure so we don't accidentally have users depending on private APIs and module imports"

Another example I noticed is someone importing `lecun_normal` and `zeros` initializers from [`linen.linear`](https://github.com/google/flax/blob/main/flax/linen/linear.py#L21) instead of `linen.initializers`.