* ASP-based solver: improve reusing nodes with gcc-runtime
This PR skips emitting dependency constraints on "gcc-runtime",
for concrete specs that are considered for reuse.
Instead, an appropriate version of gcc-runtime is recomputed
considering also the concrete nodes from reused specs.
This ensures that root nodes in a DAG have always a runtime
that is at a version greater or equal than their dependencies.
* Add unit-test for view with multiple runtimes
* Select latest version of runtimes in views
* Construct result keeping track of latest
* Keep ordering stable, just in case
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