goose: support provider cleanup functions

Documented and updated appropriate providers to reflect.

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

internal/goose/testdata/PartialCleanup/foo/foo.go

@@ -0,0 +1,51 @@
+package main
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+)
+
+var (
+	cleanedFoo = false
+	cleanedBar = false
+)
+
+func main() {
+	_, cleanup, err := injectBaz()
+	if err == nil {
+		fmt.Println("<nil>")
+	} else {
+		fmt.Println(strings.Contains(err.Error(), "bork!"))
+	}
+	fmt.Println(cleanedFoo, cleanedBar, cleanup == nil)
+}
+
+type Foo int
+type Bar int
+type Baz int
+
+//goose:provide Foo
+func provideFoo() (*Foo, func()) {
+	foo := new(Foo)
+	*foo = 42
+	return foo, func() { *foo = 0; cleanedFoo = true }
+}
+
+//goose:provide Bar
+func provideBar(foo *Foo) (*Bar, func(), error) {
+	bar := new(Bar)
+	*bar = 77
+	return bar, func() {
+		if *foo == 0 {
+			panic("foo cleaned up before bar")
+		}
+		*bar = 0
+		cleanedBar = true
+	}, nil
+}
+
+//goose:provide Baz
+func provideBaz(bar *Bar) (Baz, error) {
+	return 0, errors.New("bork!")
+}