go.benchmarks: fix garbage benchmark after src/pkg change

Possibly this code should use go/imports, but this should get the benchmark working again.

LGTM=bradfitz
R=dvyukov, adg, bradfitz
CC=golang-codereviews
https://golang.org/cl/142770043
diff --git a/garbage/garbage.go b/garbage/garbage.go
index 816adb5..31c3fdb 100644
--- a/garbage/garbage.go
+++ b/garbage/garbage.go
@@ -105,7 +105,7 @@
 // parsePackage parses and returns net/http package.
 func parsePackage() ParsedPackage {
 	pkgname := "http"
-	dirpath := runtime.GOROOT() + "/src/pkg/net/http"
+	dirpath := runtime.GOROOT() + "/src/net/http"
 	// filter function to select the desired .go files
 	filter := func(d os.FileInfo) bool {
 		if isPkgFile(d) {