Relocate MergePlugin package

Relocate the MergePlugin from org.springframework.build.gradle.merge
to org.springframework.build.gradle.

buildSrc/src/main/groovy/org/springframework/build/gradle/MergePlugin.groovy

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-package org.springframework.build.gradle.merge
+package org.springframework.build.gradle
 
 import org.gradle.api.*
 import org.gradle.api.artifacts.Configuration

buildSrc/src/main/resources/META-INF/gradle-plugins/merge.properties

@@ -1 +1 @@
-implementation-class=org.springframework.build.gradle.merge.MergePlugin
+implementation-class=org.springframework.build.gradle.MergePlugin