RFR: JDK-8261518: jpackage looks for main module in current dir when there is no module-path

[Andy Herrick]

when the app modules have already been jlinked with the runtime, and there is no need for module-path, jpackage was acting as if the module-path was "." and picking up jars in the current directory.

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Commit messages:
 - JDK-8261518: jpackage looks for main module in current dir when there is no module-path
 - JDK-8261518: jpackage looks for main module in current dir when there is no module-path

Changes: https://git.openjdk.java.net/jdk/pull/2781/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2781&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261518
  Stats: 5 lines in 1 file changed: 1 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2781.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2781/head:pull/2781

PR: https://git.openjdk.java.net/jdk/pull/2781