cmd/go: fix hang in workspaces

golang.org/cl/365234 incorrectly had pruningForGoVersion always return
workspace pruning instead of just returning workspace pruning at the top
level, which broke the proper determination of pruning for dependency
packages. Fix that code, and also fix a hang that resulted because the
module loading code keeps loading dependencies until it reaches a pruned
module or an unpruned module it already saw, so it could get stuck in a
cycle. 

Change-Id: I8911f7d83aaee5870c43ef0355abbd439f15d4f7
Reviewed-on: https://go-review.googlesource.com/c/go/+/366775


Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>