Spec: prefer a splice-specific method to `__len__` (#47585)

Automatic splicing say `Spec` grow a `__len__` method but it's only used
in one place and it's not clear the semantics are useful elsewhere. It also
runs the risk of Specs one day being confused for other types of containers.

Rather than introduce a new function for one algorithm, let's use a more
specific method in the splice code.

- [x] Use topological ordering in `_resolve_automatic_splices` instead of 
      sorting by node count
- [x] delete `Spec.__len__()` and `Spec.__bool__()`

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Signed-off-by: Todd Gamblin <tgamblin@llnl.gov>
Co-authored-by: Greg Becker <becker33@llnl.gov>
Co-authored-by: Massimiliano Culpo <massimiliano.culpo@gmail.com>