RFR: JDK-8248254: jpackage fails if app module is in external runtime

[Alexey Semenyuk]

Please review fix [2] for jpackage bug [1].

Fix jpackage code to be able to locate app module if it is linked in 
external runtime. The suggested fix only verifies if app module exists 
in external runtime. It doesn't extract version and main class from 
modules in exetrnal runtime. To address this shortcoming [3] CR was 
filed. Corresponding jtreg created and added to problem list.

- Alexey

[1] https://bugs.openjdk.java.net/browse/JDK-8248254

[2] http://cr.openjdk.java.net/~asemenyuk/8248254/webrev.00

[3] https://bugs.openjdk.java.net/browse/JDK-8248418