This replaces "nix-instantiate --eval". The result is evaluated
strictly since this seems more useful.

This allows commands like 'nix path-info', 'nix copy', 'nix verify'
etc. to work on arbitrary installables. E.g. to copy geeqie to a
binary cache:

  $ nix copy -r --to file:///tmp/binary-cache nixpkgs.geeqie

Or to get the closure size of thunderbird:

  $ nix path-info -S nixpkgs.thunderbird

So "nix path-info ./result" now works.

Thus

  $ nix build -f foo.nix

will build foo.nix.

And

  $ nix build

will build default.nix. However, this may not be a good idea because
it's kind of inconsistent, given that "nix build foo" will build the
"foo" attribute from the default installation source (i.e. the
synthesis of $NIX_PATH), rather than ./default.nix. So I may revert
this.

So for instance "nix copy --to ... nixpkgs.hello" will build
nixpkgs.hello first. It's debatable whether this is a good idea. It
seems desirable for commands like "nix copy" but maybe not for
commands like "nix path-info".