Fix a package tangle that was introduced when we added cache bypass to
`SpringIterableConfigurationPropertySource`. Ideally we should have
been able to depend on `env` from `context` but unfortunately the
`EnvironmentPostProcessor` interface references `SpringApplication`
which needs to use the Binder.

The `isImmutable` method has now been moved to `OriginLookup` which
removes the immediate tangle.

Closes gh-18393