* Restore PackageBase class, and modify only ASP
This prevents a noticeable slowdown in concretization
due to the number of directives involved.
* Fix issue with 'clang' being preferred to 'gcc',
due to runtime version weights
* Constraints on runtimes are declared by compilers
The declaration of available runtime versions, and of
their compatibility constraints are in the associated
compiler class.
Co-authored-by: Harmen Stoppels <harmenstoppels@gmail.com>
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