cmd/go/internal/modload: refactor pathInModuleCache
    
    I found the control flow of this function a bit tricky to reason about
    due to nesting and interaction between conditions and iteration. This
    change factors out a helper function that can return early instead of
    mixing conditionals and 'continue' statements.
    
    Also remove the (unused) ModuleUsedDirectly function.
    
    For #36460
    
    Change-Id: I60a2a5a1b32989e5a17a14e1a8c858b280cda8f2
    Reviewed-on: https://go-review.googlesource.com/c/go/+/251998
    Run-TryBot: Bryan C. Mills <bcmills@google.com>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: Jay Conrod <jayconrod@google.com>
    Reviewed-by: Michael Matloob <matloob@golang.org>