Rework main build POM to be an aggregator pom that does not inherit
from any parent. Introduce new spring-boot-dependencies module to
act as a parent for both spring-boot-starter-parent and
spring-boot-parent.
* For a jar deployment add classpath:static/index.html
(works via Spring MVC mapping)
* For a war the same thing works, but so does adding
index.html to src/main/webapp (works via container
default servlet)
[Fixes#54092261] [bs-252]
