move doclint to jdk.javadoc module

doclint is currently in jdk.compiler/com.sun.tools.doclint

It was placed there during modularization because of direct references from java.

Arguably, a better arrangement would be to move doclint to the jdk.javadoc module, and make it available to javac via a private JDK service.

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One minor issue: jshell makes some questionable/inappropriate direct references to doclint internal definitions that will need to be addressed. These are:
1. simple use of selected HtmlTag enum constants. (Easy enough to fix)
2. use of a mapping from `Entity` to char value. (Bigger deal; may want to expose mapping in `DocTrees`, such as `char getValue(Entity)` etc) JDK-8236142