cmd/go: cache results of HTTP requests done during meta tag discovery

cmd/go: cache results of HTTP requests done during meta tag discovery

Previously, running

  $ go get -u -v golang.org/x/tools/cmd/godoc

would results in dozens of HTTP requests for

  https://golang.org/x/tools?go-get=1

once per package under x/tools.

Now it caches the results. We still end up doing one HTTP request for
all the packages under x/tools, but this reduces the total number of
HTTP requests in ~half.

This also moves the singleflight package back into an internal
package. singleflight was originally elsewhere as a package, then got
copied into "net" (without its tests). But now that we have internal,
put it in its own package, and restore its test.

Fixes #9249

Change-Id: Ieb5cf04fc4d0a0c188cb957efdc7ea3068c34e3f
Reviewed-on: https://go-review.googlesource.com/8727
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>