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A while back we discovered that the dependencies test allowed
arbitrary dependencies for packages we forgot to list.
To stop the damage we added a grandfathered list and fixed
the code to expect unlisted packages to have no dependencies.

This CL replaces the grandfathered list with some more
careful placement of dependency rules.

Thankfully, there were no terrible inversions.

Fixes #10487.

Change-Id: I5a6f92435bd2c66c47ec8ab629edbd88b189f028
Reviewed-on: https://go-review.googlesource.com/12575

Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>