cmd/go/internal/modload: split updateRoots into separate functions for updating and tidying

In CL 293689, I fused the mvs.Reqs calls that were formerly in MinReqs
and TidyBuildList into a single function, updateRoots, in the hope
that it expressed a fundamental operation. As I have been working on
the lazy equivalents, I have come to realize that these functions are
deeply related but fundamentally different.

In order to help me reason about the two different roles, I am making
the two functions separate once more, but leaving them colocated in
the code.

For #36460

Change-Id: I851d6d81fbfd84f39411e0d076ee72a9909c60ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/310629


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