Replace pulumi-java-gen with pulumi package gen-sdk

[Frassle]

Still WIP

This replaces the use of pulumi-java-gen with pulumi package gen-sdk, the new and modern interface to codegen.

gen-sdk does make some assumptions about overlay and output paths containing the language name, so I've had to move everything in provider/pkg/gen/java-templates down one level into a nested java directory (alternatively we could have renamed java-templates to just java).

Also without the --build option to use on the command line we need to set the gradle-nexus options directly in the schema.

Finally java sdk gen doesn't set its own default dependencies, it was only the pulumi-java-gen command that filled in defaults correctly. That feels like a java codegen bug that should be fixed before we merge this so we don't have to add all the default dependencies in the schema directly (the PR currently does this).