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/server.go b/godoc/server.go
index 900f9b7..e81c673 100644
--- a/godoc/server.go
+++ b/godoc/server.go
@@ -253,7 +253,7 @@
 
 	// special cases for top-level package/command directories
 	switch tabtitle {
-	case "/src/pkg":
+	case "/src":
 		title = "Packages"
 		tabtitle = "Packages"
 	case "/src/cmd":
@@ -262,7 +262,6 @@
 	}
 
 	// Emit JSON array for type information.
-	// TODO(adonovan): display the h.c.Analysis.Status() message in the UI.
 	pi := h.c.Analysis.PackageInfo(relpath)
 	info.CallGraphIndex = pi.CallGraphIndex
 	info.CallGraph = htmltemplate.JS(marshalJSON(pi.CallGraph))
@@ -501,7 +500,7 @@
 
 	var buf bytes.Buffer
 	if pathpkg.Ext(abspath) == ".go" {
-		// Find markup links for this file (e.g. "/src/pkg/fmt/print.go").
+		// Find markup links for this file (e.g. "/src/fmt/print.go").
 		fi := p.Corpus.Analysis.FileInfo(abspath)
 		buf.WriteString("<script type='text/javascript'>document.ANALYSIS_DATA = ")
 		buf.Write(marshalJSON(fi.Data))