Move `DefaultPropertiesPropertySource` from
`org.springframework.boot` to `org.springframework.boot.env` since
it's not directly tied to `SpringApplication`.
See gh-47232
Move bootstrap code from `org.springframework.boot` to
`org.springframework.boot.bootstrap` and make them a foundational
layer.
This move helps reduce `org.springframework.boot.context.config`
dependencies to `org.springframework.boot`.
See gh-47232
Phillip Webb
Moritz Halbritter
Andy Wilkinson
Stéphane Nicoll