Add missing package infos (#1980)

Author: gnodet

api/maven-api-core/src/main/java/org/apache/maven/api/plugin/annotations/package-info.java

@@ -0,0 +1,5 @@
+// CHECKSTYLE_OFF: RegexpHeader
+/**
+ * Maven Plugin Annotations.
+ */
+package org.apache.maven.api.plugin.annotations;

api/maven-api-core/src/main/java/org/apache/maven/api/plugin/package-info.java

@@ -0,0 +1,5 @@
+// CHECKSTYLE_OFF: RegexpHeader
+/**
+ * Maven Plugin API.
+ */
+package org.apache.maven.api.plugin;