Concrete specs that are already installed or that come from a buildcache
may have compilers and variant settings that we do not recognize, but that
shouldn't prevent reuse (at least not until we have a more detailed compiler
model).
- [x] make sure compiler and variant consistency rules only apply to
built specs
- [x] don't validate concrete specs on input, either -- they're concrete
and we shouldn't apply today's rules to yesterday's build
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