cmd/golangorg: make zip contents seekable
cmd/golangorg: make zip contents seekable

https://golang.org/misc/cgo/stdio/testdata/fib.go fails right now
because the http.FileServer wants to use Seek to find out the file size.
Make that work by introducing an FS wrapper that enables seeking
in an in-memory copy of the file content.

Fixes golang/go#46809.

Change-Id: I353905310dc74594e54e0181dc821a97992b8da7
Reviewed-on: https://go-review.googlesource.com/c/website/+/329249
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Trust: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website