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
diff --git a/godoc/dirtrees.go b/godoc/dirtrees.go
index 5d35e10..c38d3b2 100644
--- a/godoc/dirtrees.go
+++ b/godoc/dirtrees.go
@@ -77,7 +77,7 @@
 	haveSummary := false
 
 	if hook := b.c.SummarizePackage; hook != nil {
-		if summary, show0, ok := hook(strings.TrimPrefix(path, "/src/pkg/")); ok {
+		if summary, show0, ok := hook(strings.TrimPrefix(path, "/src/")); ok {
 			hasPkgFiles = true
 			show = show0
 			synopses[0] = summary