internal/pkgbits: add Index type

internal/pkgbits: add Index type

Element indices are very common in the pkgbits API, so introduce a new
defined type to help make that clearer.

Change-Id: I8f9493e2335601c740eb403d1fdcd11183122907
Reviewed-on: https://go-review.googlesource.com/c/go/+/407435
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>