nix: Remove special handling of .drv files

This makes 'nix copy' and 'nix path-info' work on .drv store
paths. Removing special treatment of .drv files seems the most
future-proof approach given the possible removal of .drv files in the
future.

Note that 'nix build' will still build (rather than substitute) .drv
paths due to the unfortunate overloading in Store::buildPaths().