goose: support provider cleanup functions

Documented and updated appropriate providers to reflect.

Reviewed-by: Tuo Shan <shantuo@google.com>

internal/goose/testdata/Cleanup/foo/foo.go

@@ -0,0 +1,32 @@
+package main
+
+import "fmt"
+
+func main() {
+	bar, cleanup := injectBar()
+	fmt.Println(*bar)
+	cleanup()
+	fmt.Println(*bar)
+}
+
+type Foo int
+type Bar int
+
+//goose:provide Foo
+func provideFoo() (*Foo, func()) {
+	foo := new(Foo)
+	*foo = 42
+	return foo, func() { *foo = 0 }
+}
+
+//goose:provide Bar
+func provideBar(foo *Foo) (*Bar, func()) {
+	bar := new(Bar)
+	*bar = 77
+	return bar, func() {
+		if *foo == 0 {
+			panic("foo cleaned up before bar")
+		}
+		*bar = 0
+	}
+}