Add group discriminant in case of conflict

Prior to this commit, the repackage goal silently ignored the case of
two libraries having the same name and version but a different group.
As a result, the second library was overwriting the first one in the
repackaged jar.

This commit adds support for custom Library names and updates the
Maven and Gradle plugins so that the name includes the group ID
when there would otherwise be a duplicate.

Fixes gh-1475

Author: snicoll

spring-boot-samples/spring-boot-sample-simple/application.properties

@@ -0,0 +1 @@
+name: Phil