go.tools/godoc/analysis: convert path to url (fixes windows build)

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/94440043

godoc/analysis/analysis.go

@@ -388,7 +388,7 @@ func Run(pta bool, result *Result) {
 			for _, root := range roots {
 				rel := strings.TrimPrefix(abs, root)
 				if len(rel) < len(abs) {
-					a.path2url[abs] = "/src/pkg/" + rel
+					a.path2url[abs] = "/src/pkg/" + filepath.ToSlash(rel)
 					continue nextfile
 				}
 			}