cmd/go/internal/modindex: eliminate duplicated vars

cmd/go/internal/modindex: eliminate duplicated vars

There is no need to have two sets of the same slices.

The names are chosen to be the same as in ../imports.

Change-Id: I92dca46127e25b5226cce3cc11749cd5957ad609
Reviewed-on: https://go-review.googlesource.com/c/go/+/718800
Reviewed-by: Sean Liao <sean@liao.dev>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Sean Liao <sean@liao.dev>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>