math/rand: refactor to delay allocation of global source

math/rand: refactor to delay allocation of global source

This sets up for delaying the decision of which seed to use,
but this CL still keeps the original global Seed(1) semantics.

Preparation for #54880.

Change-Id: Ibfa9d50ec9023aa755a83852e55168fa7d24b115
Reviewed-on: https://go-review.googlesource.com/c/go/+/443057
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>