java -jar should run in module mode if the jar is a mdular jar

[mark.reinhold at oracle.com]

2016/12/5 6:15:48 -0800, tjwatson at us.ibm.com:
> This makes sense to me,

I agree that it makes sense, though it does take away the ability to run
a modular JAR easily as an unnamed module -- I don't know how important
that is to anyone.

>                         but then I wonder if the main class is specified 
> in the module-info.class as some type of service or if you continue to 
> specify that in the jar manifest.

If we do implement this then the logical approach would be to interpret
the `MainClass` attribute of the `module-info.class` file.

- Mark