All package sets are simultaneously updated to accommodate changes to
package-build. Due to new restrictions in package-build, all packages using
`melpaBuild` must now provide a recipe file, even those packages which are not
included in upstream MELPA.
Building packages requires package-build.el from Melpa, but installing
packages only requires package.el. Packages from ELPA are already built,
so there is no need to involve package-build.el.
When building a package from a Melpa recipe file, get the Emacs package
name from the recipe. Nix is more restrictive about packages names than
Emacs, so the Nix name for a package is sometimes different.
