Allow doc-files for module-info.java

We can create a doc-files directory near a Java source file and the javadoc of that class can link to files inside doc-files. However, the doc-files near module-info.java is not copied by javadoc.

I can understand the problem here. All module-name-summary.html files are in the same directory, which means their separate doc-files will be merged into one.

Currently I can put doc-files into a package and put a deep link in module-info.java, but I am not sure if the path of module-name-summary.html would change in the future.