Bug#841491: Inject A Task for Generating Maven POMs

[殷啟聰]

I adjusted the indentation to 4 spaces and sorted the imports. Looks
like Atom's sort-lines plugin ignores upper/lower cases by default.

A large number of packages builds multiple binary packages, where the
maintainer need to specify which JARs should be installed to which
binary packages. For now it is not clear which way is the best to
handle this situation, so maybe we can think about generating
$package.pom afterwards.

Finally, I think the `:clean` task simply removes the entire
$buildDir, so maybe we don't need to declare the removal actions? I
tested the plugin on various packages and `debuild clean` worked
perfectly.