Improve hit-rate on buildcaches (#43272)

* Relax compiler and target mismatches

The mismatch occurs on an edge. Previously it was assigned
the parent priority, now it is assigned the child priority.

This should make reuse from buildcaches or store more likely,
since most mismatches will be counted with "reused" priority.

* Optimize version badness for runtimes at very low priority

We don't want to e.g. switch other attributes because we
cannot reuse an old installed runtime.

* Optimize runtime attributes at very low priority

This is such that the version of the runtime would
not influence whether we should reuse a spec.

Compiler mismatches are considered for runtimes,
to avoid situations where compiling foo%gcc@9
brings in gcc-runtime%gcc@13 if gcc@13 is among
the available compilers

* Exclude specs without runtimes from reuse

This should ensure that we do not reuse specs that
could be broken, as they expect the compiler to be
installed in a specific place.

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