[foreign] RFR 8221154: jextract should generate java source files

[Sundararajan Athijegannathan]

Please review.

This is initial change for moving jextract towards source generation. 
The option --src-dump-dir will most likely be changed/removed later. It 
is a development/debug option for now. Reasons for source instead of 
direct .class generation are documented in the bug report.

Bug: https://bugs.openjdk.java.net/browse/JDK-8221154
Webrev: https://cr.openjdk.java.net/~sundar/8221154/webrev.00/

Thanks,
-Sundar