Allow for multiple `child(..., get_if_exists: true)` in a single `:spec` action.

[varsill]

Currently, the following :spec action would fail:

[child(:source, Source, get_if_exists: true), child(:source, Source, get_if_exists: true) |> child(:sink, Sink)]

due to "duplicated children names in spec" error.
There is nothing that prevents us from handling this spec correctly unless the child definition (module or a struct) differs among child(..., get_if_exists: true) function invocations.
It could simplify writing specs in cases where some of the spec branches which originate in the same output pads component are spawned conditionally.

[mat-hek]

As we discussed internally, it would be nice to address this pain point, but the solution isn't obvious.

There is nothing that prevents us from handling this spec correctly unless the child definition (module or a struct) differs among child(..., get_if_exists: true) function invocations.

I generally agree, but it would be inconsistent with how get_it_exists behaves now (with different specs) - it doesn't require the child definition to be the same. IMO, it would be better to allow different child definitions and always pick the first one (there's order within the single spec too), but also always warn when the definitions differ.

I think the original problem is that we define the 'same' child multiple times, so that some definitions can be ignored. To avoid this problem, we could introduce a new concept - lazy_child (better ideas for the name are welcome), that could be used as follows:

# Create an opaque reference to a demuxer, no actual child is created at this point
demuxer = lazy_child(:demuxer, Some.Demuxer)

# When this spec is returned, the demuxer is created according to the spec passed to lazy_child
spec = child(Some.Source) |> child(demuxer) |> child(Some.Sink)

# When this spec is returned, the demuxer already exists, so it's only linked to Another.Source
# demuxer can be stored in the state and returned in a subsequent callback
spec2 = child(Another.Source) |> child(demuxer)

This concept could replace get_if_exists (we could deprecate it), and we wouldn't have any ignored definitions. The downside is that it brings in some new complexity.