[foreign] RFR 8223185: Consolidate jextract output logic in Writer

[Jorn Vernee]

Hi,

I have been experimenting with a plugin based on jextract's new source 
code generation, for my earlier idea to generate LayoutType constants in 
various generated classes [1].

I ran into a problem using --src-dump-dir. Since the dumping is done 
inline in the JavaSourceFactory* classes this meant the the changes from 
the plugin were not being shown in the output.

It seems good to move the source output logic to Writer, since then we 
can do the source output once and for all in Main, after the jextract 
run. I've also moved the JModWriter and JarWriter calls into Writer, so 
that we have 1 main interface for doing output (Writer).

Please review the following:

Bug: https://bugs.openjdk.java.net/browse/JDK-8223185
Webrev: 
http://cr.openjdk.java.net/~jvernee/panama/webrevs/8223185/webrev.00/

Thanks,
Jorn

[1] : https://bugs.openjdk.java.net/browse/JDK-8220063