RFR: 8216170: java.lang.IllegalArgumentException: directories not supported

[Jonathan Gibbons]

Please review a medium small update to fix a nasty crash in a somewhat 
obscure set of circumstances.

The root cause is that javadoc was treating any standalone option ending 
in ".java" that named an existing file as a source file, even when the 
option was actually a package name. Since it is not common to have 
directories ending in ".java", the problem has not arisen before now. 
The unusual circumstances that triggered the bug were a combination of 
the package existing in an enclosing module with the same name, and 
javadoc output being written into the current directory. The net effect 
is that the first time javadoc was run, it ran OK, but on subsequent 
runs, javadoc confused the package name of the command line with the 
directory created for the module's documentation and named after the module.

The fix is trivial.  The test case recreates the test case scenario.

JBS: https://bugs.openjdk.java.net/browse/JDK-8216170
Webrev: http://cr.openjdk.java.net/~jjg/8216170/webrev.00/index.html

-- Jon