This commit moves a lot of the heavy lifting out of callPackage and back
into regular import, following conventions from upstream Nixpkgs. It
allows for a clearer and less magic definition of new packages,
particularly by downstream, as well, by exposing the *Build package
callers in the attribute set.
The unpacking logic for cargoDeps is changed so that the filename
doesn't implicitly have a file suffix appended to it, so we have to
specify that it's a tarball manually. The output hash is updated to
match.
Ones of the changes from 0.5.0 is depending on the system fontconfig,
and one of the ligature changes forces the use of the system freetype.
One of the Rust libraries depends on the C++ stdlib.
At some point the build process stopped bundling the OpenGL libraries,
so add those back in too.
The old method was having the alacritty build copy-pasted into the repo
and making minor changes. This worked but isn't really feasible as it
doesn't incoroprate upstream (build) changes without noticing.
This commit finally works out how to override buildRustPackage to set
the Cargo dependency.
