cmd/go: don't panic when explaining lost upgrades due to downgrades
If a user runs 'go get mod@vers' where the module transitively
requires itself at a newer version, 'go get' attempts to perform a
downgrade, which necessarily excludes the requested version of the
module.
Previously, we called mvs.BuildList with the requested module
version as the target. This panicked because BuildList doesn't allow
the target module (typically the main module) to require a newer
version of itself.
With this change, when we lose an upgrade due to a downgrade, we call
mvs.BuildList through a wrapper that treats the lost module version as
requirement of a synthetic root module, rather than the target
module. This avoids the panic.
This change also starts reporting errors when an upgraded module is
lost entirely (downgrades caused the module to be completely removed
from the build list).
Fixes #31491
Change-Id: I70ca261c20af7553cad2d3b840a1eaf3d18a4191
Reviewed-on: https://go-review.googlesource.com/c/go/+/177602
Run-TryBot: Jay Conrod <jayconrod@google.com> Reviewed-by: Bryan C. Mills <bcmills@google.com>