Add mechanism for enforcing ordering of wheelhouse install

The dependencies in `wheelhouse/` are installed in an "undefined" order (it comes from a glob, so I'm pretty sure it's alphabetical). Most of the time, pip will resolve the dependencies properly so that's fine. However, it occasionally fails for things that are not strictly dependencies but still need to be installed in a particular order.

My current thought is to have a `wheelhouse_order` layer option that is a list of packages from the wheelhouse (just the package names, not full requirement specifiers nor file names). These will be installed from the wheelhouse, in order. Once they are all processed, any remaining items in the wheelhouse are installed in the existing order. That way, only "need to be installed first" items must be listed, and multiple items can be given a defined order.

Start by locating the layer logic that installs dependencies from `wheelhouse/` and checking how layer options are defined and read. Trace the existing installation order and its tests, if present. Done means a `wheelhouse_order` option can specify package names in order while remaining wheelhouse items retain the existing installation behavior.

Written by the indexing model from the issue text.