cmd/pack: avoid ./ import in test (fix Windows build)

cmd/pack: avoid ./ import in test (fix Windows build)

It is possible to use ./ imports on Windows but it
requires some extra command-line work
('go build' does this automatically, but we can't use 'go build' here).

Instead, use an ordinary import and -I/-L, which are easier to use.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/89040043