internal/coverage: use io.Seek* constants

internal/coverage: use io.Seek* constants

Use the io.Seek* constants instead of their deprecated os.SEEK_*
counterparts.

Change-Id: Ie899f9cf286de7304a394d9a2a091a55937542ad
Reviewed-on: https://go-review.googlesource.com/c/go/+/453576
Reviewed-by: Than McIntosh <thanm@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>