The concretizer can fail with `reuse:true` if a buildcache or installation contains a
package with a dependency that has been renamed or deleted in the main repo (e.g.,
`netcdf` was refactored to `netcdf-c`, `netcdf-fortran`, etc., but there are still
binary packages with dependencies called `netcdf`).
We should still be able to install things for which we are missing `package.py` files.
`Spec.inject_patches_variant()` was failing this requirement by attempting to look up
the package class for concrete specs. This isn't needed -- we can skip it.
- [x] swap two conditions in `Spec.inject_patches_variant()`
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