x/pkgsite: go.mod for cmd is not the right go.mod

[hyangah]

https://pkg.go.dev/cmd is pointing to the src/go.mod
https://go.googlesource.com/go/+/go1.15.8/src/go.mod

It needs to be src/cmd/go.mod
https://go.googlesource.com/go/+/go1.15.8/src/cmd/go.mod

[jba]

This is a consequence of pkgsite treating everything under src as a single module, regardless of internal module divisions.
@julieqiu perhaps we should revisit that decision.