Nixlang-centric definition of package attribute

[roberth]

Nothing prevents an expression author from doing the following when putting a package in an attribute:

• return a source
• replace an output by the output of another derivation
• not return a drvPath

See

• Redefine outputs in terms of language-level "package", not necessarily store-level derivation (RFC-92, and multi-drv packages, docs) NixOS/nix#6507