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/NoImplicitInterface/foo/foo.go

@@ -16,7 +16,6 @@ func (b Bar) Foo() string {
 	return string(b)
 }
 
-//goose:provide
 func provideBar() Bar {
 	return "Hello, World!"
 }