The main purpose of this change is to make layer grouping more robust.
Instead of using a heuristic based on whether a library is in the same
`organization` as the build (which is not always what you want, some
builds may have sub projects with different organizations, and some
builds may depend on external dependencies from the same organization),
it checks whether the library in question is present in
`projectDependencyArtifacts` (ie, whether its a dependency built by
this build), if it is, then it goes in a higher layer.

Doing this change however required changing `dockerLayerGrouping` to be
a `TaskKey` instead of a `SettingKey`, which is a non binary compatible
change. So, `dockerLayerGrouping` has been deprecated, and a new
`TaskKey`, `dockerGroupLayers` has been introduced.

Since a new task has been introduced, I also took advantage of that to
improve the types, instead of just passing the destination path, which
requires you to know how destination paths are built to check if a
particular file should be in there, it passes the whole mapping, ie, the
source file and the destination path, which means that you can match
against the source file (which is what `projectDependencyArtifacts`
gives you) as well. Also, I changed the type from a function that
returns an `Option` to a `PartialFunction`, this makes composing it, to
introduce new mappings, far simpler, as `PartialFunction[_, _]` composes
trivially compared to `Function[_, Option[_]]`.