Motivation

tex-gyre: use callPackages

Using callPackages instead of callPackage adds an override attribute to
each individual package instead of the set. Each member of the set is then just
a package so that lib.attrValues tex-gyre gives a list of packages that can be
installed.

tex-gyre-*-math: Move to tex-gyre-math package set

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.

Testing

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

CC: @siddharthist