cmd/go: simplify and document lazy-loading test cases

I decided to add package and module diagrams to the test cases to make
them easier to follow.

While adding those diagrams, I noticed some strong similarities among
a couple of the graphs, so I consolidated those cases (and deleted the
redundant tests).

For #36460

Change-Id: Id6cd04fc871379b83851c2d1af89ea9296a0f3e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/251997


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>