The Git plugin was primarily being used to provide version information
that Boot's maven plugin can add into the MANIFEST.MF of the uber-jars
that it creates under the Spring-Boot-Commit-Id attribute.
This commit removes the Git plugin from Boot's own projects, but
leaves it in the spring-boot-starter-parent for use by Spring
Boot-based applications.
The attribute in the uber-jars' MANIFEST.MF has been replaced with a
Spring-Boot-Version attribute. The value of this attribute is the
implementation version of Repackager class's package.
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.
