Because of the way abstract variants are implemented, the following
spec matrix does not work as intended:
```
matrix:
- [foo]
- [bar=a, bar=b]
exclude:
- bar=a
```
because abstract variants always satisfy any variant of the same
name, regardless of values.
This PR converts abstract variants to whatever their appropriate
type is before running satisfaction checks for the excludes clause
in a matrix.
fixes#16841
2795414a80