godoc/vfs: add io/fs adapter
godoc/vfs: add io/fs adapter

Change-Id: Iaeea108ce2803d3ee1ece5d2d67fbe353576fbd7
Reviewed-on: https://go-review.googlesource.com/c/tools/+/291669
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Trust: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools