tour: serve from cmd/golangorg as go.dev/tour

tour.golang.org is the last independent server in this repository
and almost the last golang.org domain with user-facing content.
(talks.golang.org remains.)

We need to keep golang.org/x/website/tour as a runnable binary,
but move the logic into internal/tour so it can be used from both
the tour binary and cmd/golangorg.

After this is deployed, we will need to delete the current tour app
from the golang-org project, and then requests for tour.golang.org
will default to the main server (cmd/golangorg).

This will make the tour available in China for the first time,
at golang.google.cn/tour.

Change-Id: I0f025d8ae89e12489d26bb3bc380a833eeb57bcc
Reviewed-on: https://go-review.googlesource.com/c/website/+/365100
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Jamal Carvalho <jamal@golang.org>

_content/tour/concurrency.article

@@ -96,7 +96,7 @@ Use a `default` case to try a send or receive without blocking:
 
 There can be many different binary trees with the same sequence of values stored in it. For example, here are two binary trees storing the sequence 1, 1, 2, 3, 5, 8, 13.
 
-.image /content/img/tree.png
+.image /tour/static/img/tree.png
 
 A function to check whether two binary trees store the same sequence is quite complex in most languages. We'll use Go's concurrency and channels to write a simple solution.
 

_content/tour/content_test.go

@@ -43,7 +43,7 @@ func TestContent(t *testing.T) {
 		}
 		t.Run(path, func(t *testing.T) {
 			t.Parallel()
-			if err := testSnippet(t, path, scratch); err != nil {
+			if err := testSnippet(t, filepath.ToSlash(path), scratch); err != nil {
 				t.Errorf("%v: %v", path, err)
 			}
 		})