cmd/go: 'go mod why -m' and 'go mod graph' should report modules in the compatibility graph. #46366
Description
To ease the migration path for pruned module graphs in Go 1.17 (#36460), we added a -compat flag to go mod tidy (#46141) to preserve additional checksums for Go 1.16.
Ideally, go mod why -m (and perhaps go mod graph?) should also report why those checksums are needed, since they're a bit mysterious otherwise.