Simplify new doclet packages

The new javadoc API has 5 classes in 3 packages.

langtools/src/jdk.javadoc/share/classes/jdk/javadoc/doclet/DocletEnvironment.java
langtools/src/jdk.javadoc/share/classes/jdk/javadoc/doclet/package-info.java
langtools/src/jdk.javadoc/share/classes/jdk/javadoc/doclet/Doclet.java
langtools/src/jdk.javadoc/share/classes/jdk/javadoc/doclet/taglet/Taglet.java
langtools/src/jdk.javadoc/share/classes/jdk/javadoc/doclet/taglet/package-info.java
langtools/src/jdk.javadoc/share/classes/jdk/javadoc/doclet/Reporter.java
langtools/src/jdk.javadoc/share/classes/jdk/javadoc/doclets/package-info.java
langtools/src/jdk.javadoc/share/classes/jdk/javadoc/doclets/StandardDoclet.java

In hindsight, and in view of the introduction of the module system, this is two packages too many. We are unlikely to add new doclets and even if we did they would either be strongly related to the StandardDoclet or would be in some other module.

We should coalesce these 3 packages into 1.