This PR fixes the issues with `%` and reused specs, due to https://github.com/spack/spack/issues/49847#issuecomment-2774640234
It does so by adding another layer of indirection, so that whenever a spec
`foo %bar` is encountered, the `%bar` part is encoded as an
`attr("build_requirement", ...)`.
Then:
1. If `foo` is a node to be built, then the build requirement implies a dependency
2. Otherwise it implies looking e.g. reused specs metadata, and ensure it matches
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Signed-off-by: Massimiliano Culpo <massimiliano.culpo@gmail.com>
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