useful for mesa, where there are a signficant number of crates
referenced via meson wrap files, but this could be generally useful for
other meson-based packages that do a similar thing for vendored
libraries.
This could be further extended in the future to add support for
git/hg/svn wraps by adding subclasses of `Wrap` that support those
metadata options.
setting `MESON_PACKAGE_CACHE_DIR` allows meson to look for tarballs for
wraps it needs automatically.
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