At least make the system start up when there's a component
with a missing resource. This is a first step towards a refactoring
so that we can deal with multiple resources for one path.
Can do this slightly later.
Refactor path_to_resource to path_to_resources, so that we can
deal with multiple resources per path there.
Refactor things so that main can be a list or missing. It still doesn't
properly handle these cases, though, but the tests still pass.
Change create_resource into create_resources. Still can only handle
a single resource at the end.
Make resources be a multiple. It's not pretty, but the tests continue
to work.
Missing main starts to work now.
Multiple mains works now too.
Slight refactoring of the name to make things clearer.
Add a few tests for depending on components with missing or
multiple mains.