Jar has a very low set of overlap with other target types, including
that jar sources *must* be .java, and no other target allows .java
sources. As such, the difficulty in crafting a useful `build_target`
invocation that allows both `jar` and anything else is high, and the
usefulness is dubious. Just use `jar()` directly instead.
This depends on the changes to make all of the jar() specific keyword
arguments be handled by typed_kwargs so that the deprecation messages
are correct and consistent.