Errors out with incremental compilation

[marquiswang]

Reproduceable by setting up a Maven project using therapi-runtime-javadoc, and then running mvn clean compile compile.

[ERROR] Caused by: javax.annotation.processing.FilerException: Attempt to reopen a file for path /Users/mwang/code/example/target/classes/com/foo/bar/ExampleClass__Javadoc.json
[ERROR]         at jdk.compiler/com.sun.tools.javac.processing.JavacFiler.checkFileReopening(JavacFiler.java:766)
[ERROR]         at jdk.compiler/com.sun.tools.javac.processing.JavacFiler.createResource(JavacFiler.java:543)
[ERROR]         at com.github.therapi.runtimejavadoc.scribe.JavadocAnnotationProcessor.createJavadocResourceFile(JavadocAnnotationProcessor.java:155)
[ERROR]         at com.github.therapi.runtimejavadoc.scribe.JavadocAnnotationProcessor.outputJsonDoc(JavadocAnnotationProcessor.java:144)
[ERROR]         at com.github.therapi.runtimejavadoc.scribe.JavadocAnnotationProcessor.generateJavadocForClass(JavadocAnnotationProcessor.java:138)
[ERROR]         at com.github.therapi.runtimejavadoc.scribe.JavadocAnnotationProcessor.generateJavadoc(JavadocAnnotationProcessor.java:118)