RFR: JDK-8189094: Change required boot jdk to JDK 9

[Martin Buchholz]

   1.

   The upgrade module path. This path contains compiled definitions of
   modules that will be observed in preference to the compiled definitions of
   any *upgradeable modules* that are present in (3) and (4). See the Java
   SE Platform for the designation of which standard modules are upgradeable.

The doc for java.lang.module says "See the Java SE Platform" but I guess
that just means checking the javadoc for each module to check whether it's
upgradeable.  It's surprising to see runtime modules tied together to the
extent of recording hashes of contents of reverse dependencies, but as you
say few developers will need to care.  This does mean you can never "drop
in" an independently built module into a jdk the way hotspot developers do
with libjvm.so.  But that's OK - --patch-module and the ability to build
from source are sufficient.