It turns out that the relative-path support never worked in the first
place.

It had been masked by the fact that we ~never invoke overlayDir with
an absolute path, which caused filepath.Rel to always return an error,
and overlayDir to always fall back to absolute paths.

Since the absolute paths seem to be working fine (and are simpler),
let's stick with those. As far as I can recall, the relative paths
were only a space optimization anyway.

Updates #28387
Updates #30316

Change-Id: Ie8cd28f3c41ca6497ace2799f4193d7f5dde7a37
Reviewed-on: https://go-review.googlesource.com/c/go/+/208481
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>