os: don't hide all methods in recursive call to io.Copy

In order to avoid a recursive call to ReadFrom, we were converting
a *File to an io.Writer. But all we really need to do is hide
the ReadFrom method. In particular, this gives us the option of
adding a WriteTo method.

For #58808

Change-Id: I20d3a45749d528c93c23267c467e607fc17dc83f
Reviewed-on: https://go-review.googlesource.com/c/go/+/475535


Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>