context: remove dependency on reflect

Make context depend on reflectlite instead of reflect in effort to
eventually make net no longer depend on unicode tables.

With this CL we're down to just:

    net -> context -> fmt -> unicode tables

The next CL can remove context -> fmt.

Updates #30440

Change-Id: I7f5df15f975d9dc862c59aa8477c1cfd6ff4967e
Reviewed-on: https://go-review.googlesource.com/c/go/+/164239


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>