* make font derivations fixed-output where applicable
* fix dead links
* `stdenv.lib` -> `lib` where `stdenv` is not involved
* remove `meta.platforms = [ unix ]` and `meta.platforms = [ linux ]` because the restriction has no sense for data packages
The `tex-gyre-*-math` fonts are moved to the `tex-gyre-math` set for consistency
with `tex-gyre` and to allow them to be easily installed together. Aliases are
created for backwards-compatibility.
