all: update to use filepath.WalkDir instead of filepath.Walk

Now that filepath.WalkDir is available, it is more efficient
and should be used in place of filepath.Walk.
Update the tree to reflect best practices.

As usual, the code compiled with Go 1.4 during bootstrap is excluded.
(In this CL, that's only cmd/dist.)

For #42027.

Change-Id: Ib0f7b1e43e50b789052f9835a63ced701d8c411c
Reviewed-on: https://go-review.googlesource.com/c/go/+/267719


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>