cmd/go: add a 'go mod why' mode to answer “why this specific version?” #28800
Description
As noted in #28795 (comment), today we can use go mod graph to figure out which module imposes a particular version of a dependency, then use go mod why to figure out why that module is included.
However, that's two queries to answer what is logically a single question: “why was this version of the module selected?” Perhaps we should add a go mod why flag to answer that question more directly.