The httpresponse.go module wants to be able to tell if a particular type t
is net/http.Response (and also net/http.Client). It does this by importing
net/http, looking up Response, and then comparing that saved type against
each t.

Instead of doing an eager import of net/http, wait until we have a type t
to ask a question about, and then just look to see if that t is http.Response.
This kind of lazy check does not require assuming that net/http is available
or will be important (perhaps the check is disabled in this run, or perhaps
other conditions that lead to the comparison are not satisfied).

Not loading these kinds of types at startup time will scale better.

Change-Id: Ibb00623901a96e725a4ff6f231e6d15127979dfd
Reviewed-on: https://go-review.googlesource.com/74353
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>