Used for custom package derivation or for scripts.

For example, to use a newer package version that is still not in nixpkgs-unstable.

Some rules about package naming:
* If it is a custom derivation of a package, end with `ManuallyDerived`
* If it is a script, start with `custom-`. The executable, package name (and file name if it is the only thing in this nix file) must be equal.