cmd/go: refer to the right package in a test

The test checks that two packages aren't non-test dependencies.
There's a copy-paste typo, however.
When net/http is unexpectedly found as a dependendency,
we instead mention the other package in the error message.

Change-Id: I3232c6252255c839e08efa048f2232c192d0fb85
Reviewed-on: https://go-review.googlesource.com/c/go/+/351372


Trust: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>