This ensures that the lock file is updated *before* evaluating it, and
that it gets updated for any nix command, not just 'nix build'.
Also, while computing the lock file, allow arbitrary registry lookups,
not just at top-level.
Also, improve some error messages slightly.
Unlike file://<path>, this allows the path to be a dirty Git tree, so
nix build /path/to/flake:attr
is a convenient way to test building a local flake.
The general syntax for an installable is now
<flakeref>:<attrpath>. The attrpath is relative to the flake's
'provides.packages' or 'provides' if the former doesn't yield a
result. E.g.
$ nix build nixpkgs:hello
is equivalent to
$ nix build nixpkgs:packages.hello
Also, '<flakeref>:' can be omitted, in which case it defaults to
'nixpkgs', e.g.
$ nix build hello
This allows using an arbitrary "provides" attribute from the specified
flake. For example:
nix build --flake nixpkgs packages.hello
(Maybe provides.packages should be used for consistency...)
