The time package has never depended on the io package until
a recent change during Go 1.7 to use the io.Seek* constants.

The go/build dependency check didn't catch this because "time" was
allowed to depend on meta package group "L0", which included "io".

Adding the "io" package broke one of Dmitry's tools. The tool is
fixable, but it's also not necessary for us to depend on "io" at all
for some constants. Mirror the constants instead, and change
deps_test.go to prevent an io dependency in the future.

Change-Id: I74325228565279a74fa4a2f419643f5710e3e09f
Reviewed-on: https://go-review.googlesource.com/22960
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>