Speed up eval by reducing data dependencies

[Chris Warburton]

Evaluating nix-config can take a while, which slows down many commands.
Since Nix is lazy, it should only evaluate what is needed, but running
commands verbosely shows lots of seemingly-random files being evaluated.

This may be due to unnecessary data dependencies, for example we might
have an attrset like:

{
  a = import foo...;
  b = bar...;
  c = baz...;
}

In this case, when we ask for (say) the 'b' parameter, the 'a' and 'c'
are ignored, 'foo' won't be imported and 'baz' won't be called. Only
'bar' will be called.

Now compare an alternative setup:

fold (x: y: x // fiddleWith y)
     {}
     [ { name = "a"; value = import foo...; }
       { name = "b"; value = bar...;        }
       { name = "c"; value = baz...;        } ]

The result is still an attrset, but it's been built up piecemeal from
each of these components. When we ask for 'b' in this case, the 'fold'
will be forced, to give the equivalent of:

{} // fiddleWith { name = "a"; value = import foo..; }
   // fiddleWith { name = "b"; value = bar...;       }
   // fiddleWith { name = "c"; value = baz...;       }

We can't get the 'b' attribute from this directly, since we don't know
which one of these parts defines it. We'll start from the last line
(since that would override anything defined before):

{} // fiddleWith { name = "a"; value = import foo..; }
   // fiddleWith { name = "b"; value = bar...;       }
   // { c = baz...;                                  }

This has forced the call to 'fiddleWith', which might cause quite a bit
of work, but *we* know (due to the way we just-so-happen to define
things) that there'll be no "b" in there. So we carry on:

{} // fiddleWith { name = "a"; value = import foo..; }
   // { b = bar...;
        c = baz...;                                  }

Now we've called 'fiddleWith' again, for the 'b' case, which again might
do a bunch of work. This time there's a 'b' attribute defined, so we can
now pick that out and do the 'bar' call as in the above example.

The question is, can we define things in a way which is more like the
first example, but still allow the names and attributes to be
calculated rather than hard-coded in an attrset literal?