cmd/go: make fewer 'go mod' commands update go.mod

'go mod graph', 'go mod vendor', 'go mod verify', and 'go mod why'
will no longer edit go.mod or go.sum.

'go mod graph', 'go mod verify', and 'go mod why' may still fetch
files and look up packages as if they were able to update
go.mod. They're useful for debugging and should still work when go.mod
is a little broken.

This is implemented in modload.setDefaultBuildMod based on command
name for now. Super gross. Sorry. This should be fixed with a larger
refactoring for #40775.

Fixes golang/go#45551

Change-Id: If5f225937180d32e9a5dd252c78d988042bbdedf
Reviewed-on: https://go-review.googlesource.com/c/go/+/336151


Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-on: https://go-review.googlesource.com/c/go/+/341933


Reviewed-by: Bryan C. Mills <bcmills@google.com>