Pass around the imported package explicitly instead of relying
on a global variable.

Unfortunately we still need a global variable to communicate to
the typechecker that we're in an import, but the semantic load
is significantly reduced as it's just a bool, set/reset in a
couple of places only.

Change-Id: I4ebeae4064eb76ca0c4e2a15e4ca53813f005c29
Reviewed-on: https://go-review.googlesource.com/38595
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>