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

[Erik Joelsson]

With JDK 9 released, it's high time to change the required boot jdk for 
building JDK 10. This time, the change wasn't as straight forward as it 
usually is.

It's currently possible to use any of JDK 8, 9 or a recent build of 10 
to boot the JDK 10 build. This support is however fragile. The most 
sensitive part is the building and running of the interim javac and 
javadoc tools, where we build the new JDK 10 versions of these tools, 
but -source/-target set appropriate for the boot jdk, so we can run them 
on the boot jdk when compiling the rest of the product.

In the current build, we compile the java source files of the modules 
java.compiler, jdk.compiler, jdk.javadoc and jdk.jdeps in the legacy 
way, using -Xbootclasspath and not including any module-info.java files. 
We then run them by using the --patch-module argument. This means we are 
running the JDK 10 classes but using the module definitions of the boot 
jdk. This works for now, but when the JDK module definitions for any of 
these modules need to change, this model will start to break.

In this patch I have tried to change this so we compile and run using 
JDK 9 module style modes. The big problem to overcome then is that 
jdk.compiler, and jdk.javadoc are not upgradeable. This means we can't 
compile new versions of these modules and override them in the boot jdk. 
This leaves us with two options: either run the interim classes in the 
unnamed module, or define a new set of interim modules, based on the 
existing modules but with new names. The first option seems simpler, but 
that would require maintaining legacy service provider definitions for 
these modules. So I chose the latter instead. The new module names have 
".interim" as suffix.

To generate the new modules, I copy the module-info.java files to a new 
gensrc dir and sed replace the module names. I also generate a new 
ToolProvider.java so that the default tools are taken from the interim 
modules.

I've made sure that jrtfs.jar is generated with --release 8 to keep 
compatibility with JDK 8.

Webrev: http://cr.openjdk.java.net/~erikj/8189094/webrev.01

Bug: https://bugs.openjdk.java.net/browse/JDK-8189094

/Erik