Review request: 8055856: checkdeps build target doesn't work for cross-compilation builds

[Mandy Chung]

JDK-8055856: checkdeps build target doesn't work for cross-compilation builds
JDK-8056113: [build] tools.jar missing modules.xml

Webrev at:
    http://cr.openjdk.java.net/~mchung/jdk9/webrevs/8055856/

This patch fixes a few things about modules.xml

1. jdeps is invoked at build time to verify the module boundaries.
    For cross-compilation, it has to run on the host system.
    jdeps is added in the langtools interim build so that it
    can run with the BOOT_JDK.  jdeps is also modified to
    support a system property to specify the path to modules.xml
    generated later.

2. The generated modules.xml is solely for jdeps to use until
    the module system is in place.  The build tool takes the
    modules.xml in the top repo and generates a new file
    to include the module membership for jdeps to use.  It
    is better to name it differently to jdeps-modules.xml
    to avoid confusion as Magnus suggests.

3. main-jars is missing the dependency of modules-xml target
    and thus jdeps in the JDK images fails.  To help catch
    this issue, jdeps now throws an exception if jdeps-modules.xml
    is missing.

thanks
Mandy