Sample packages for the tests.

demo/ is a small library mixing Scala and Java, with a resource. Each
directory under variants/ holds the debian/ files that make it a package
for one case (Scala 2.12, 2.13, 3, both 2.13 and 3, and cases that must
fail). "assemble VARIANT DEST" puts the two together; t/build.t and
debian/tests/build-library use it.
