goose: use marker functions instead of comments

To avoid making this CL too large, I did not migrate the existing goose
comments through the repository.  This will be addressed in a subsequent
CL.

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

internal/goose/testdata/InjectInput/foo/foo.go

@@ -1,6 +1,10 @@
 package main
 
-import "fmt"
+import (
+	"fmt"
+
+	"codename/goose"
+)
 
 func main() {
 	fmt.Println(injectFooBar(40))
@@ -10,12 +14,14 @@ type Foo int
 type Bar int
 type FooBar int
 
-//goose:provide Set
+var Set = goose.NewSet(
+	provideBar,
+	provideFooBar)
+
 func provideBar() Bar {
 	return 2
 }
 
-//goose:provide Set
 func provideFooBar(foo Foo, bar Bar) FooBar {
 	return FooBar(foo) + FooBar(bar)
 }