go.tools/godoc: deal with fallout from $GOROOT/src/pkg -> $GOROOT/src renaming.

To avoid breaking URLs, we redirect /src/pkg/* to /src/*.

The URL /pkg is now the "directory" /src, which triggers the
"Packages" index.

All other references to "src/pkg" are now gone,
except a number in the namespace documentation which are
probably still illustrative.

Tested: go test cmd/godoc godoc
        Manual inspection of src and src/pkg pages.
        with GOROOT and GOPATH packages
        -analysis
        /AUTHORS file URL still works

LGTM=bradfitz, adg
R=bradfitz, adg
CC=golang-codereviews
https://golang.org/cl/141770044
15 files changed