use output_directory in javac hermetic compile

[baroquebobcat]

Once #5709 has been implemented, we need to update javac hermetic compile to collect the classes directory and the context jar rather than just individual files.

[stuhood]

Note that it is likely also an option to enable-by-default and then deprecate the use-classpath-jars option. You could then remove the loose classfile output entirely.

[stuhood]

Another potential tool in the toolbelt is to add a flag to zinc to jar up the output directory after running, which would avoid needing multiple processes.

[baroquebobcat]

Oh right, we could have our pants zinc wrapper do it. That's a good idea.