XD-2010: Fix duplicate packages

Moved gemfire-server to its own subpackage.
Removed package-info from the spring-xd-test project.


Fixes the following:
```
:api
/Users/ebottard/Documents/projects/spring-xd/spring-xd-gemfire-server/src/main/java/org/springframework/xd/gemfire/package-info.java:5: warning: a package-info.java file has already been seen for package org.springframework.xd.gemfire
package org.springframework.xd.gemfire;
                              ^
javadoc: warning - Multiple sources of package comments found for package "org.springframework.xd.gemfire"
/Users/ebottard/Documents/projects/spring-xd/spring-xd-test/src/main/java/org/springframework/xd/dirt/integration/bus/package-info.java:5: warning: a package-info.java file has already been seen for package org.springframework.xd.dirt.integration.bus
package org.springframework.xd.dirt.integration.bus;
                                               ^
javadoc: warning - Multiple sources of package comments found for package "org.springframework.xd.dirt.integration.bus"
```