- `_expand_virtual_packages` now gets a candidate list and will try
all the candidates.
- Good news: If the first virtual in the list conflicts with something else in
the spec, we'll keep trying until we find a good one.
- Bad news: Only looks as far as the next normalize(); can't see
conflicts further ahead than that if they're inevitable
some other virtual expansion.
- Refactor `concretize.py` to keep all the nasty spec graph stitching in
`spec.py`. This is more similar to before externals support.
- `concretize.py` now just returns a list of candidates sorted by
ABI compatibility to `_expand_virtual_packages`, and `spec.py`
handles testing the candidates.
- Refactor the way external paths are handled in `config.py` and `concretize.py`:
- previously, `spec_externals` returned spec/path pairs. Now it
returns specs with `external` set. Makes code in `concretize.py`
more natural.
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