Specs that did not contribute any files to an env view caused a problem
where zip(specs, files grouped by prefix) got "out of sync", causing the
wrong merge map to be passed to a package's `add_files_to_view`, which
specifically caused an issue where *sometimes* bin/python ended up as a
symlink instead of a copy.
One such example is kokkos + kokkos-nvcc-wrapper, as the latter package
only provides the file bin/nvcc_wrapper, which is also added to view by
kokkos, causing kokkos-nvcc-wrapper to contribute 0 files.
The test feels a bit contrived, but it captures the problem... pkg a is
added first and has 0 files to contribute, pkg b adds a single file, and
we check if pkg b receives a merge map (and a does not).
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