Add the replace-dependency build support function.

The use case is to do a deep replacement of a dependency without rebuilding the entire tree.
For example, suppose a security hole is found in glibc and a patch released. Ideally, you'd
just rebuild everything, but that takes time, space, and CPU that you might not have, so in
the mean time you could build a safe version of, say, firefox with:

  firefox-safe = replace-dependency { drv = firefox; old-dependency = glibc; new-dependency = patched-glibc; };

Building firefox-safe will rebuild glibc, but only do a simple copy/string replacement on all other dependencies
of firefox. On my system (MBP 13" mid-2012), after a new glibc had been build building firefox took around 11 seconds.

See the comments in the file for more details.