Add shouldFail dependencies to overridden packages

[Chris Warburton]

At the moment, our overrides are monotonically increasing: package 'foo'
fails to build, so we provide 'foo.override ...' which builds
successfully.

Since we're shadowing such packages with overrides, there's no way to
tell if they're actually needed anymore! Ideally the upstreams
(developers or packagers) should be fixing bugs, making our overrides
unnecessary. If this happens, we'll just carry on overriding, never
noticing that the original now works.

To prevent this, we can use the 'shouldFail' function. For each override
we should:

 - Capture the problem with a failing derivation. If the problem is that
   the package fails to build, then use the package itself. If it's some
   runtime symptom, e.g. trying to run the resulting binaries doesn't
   work, then make a simple test derivation which depends on the
   original package, runs the binary and dies due to the problem.
 - Pass this failing derivation to 'shouldFail', and add as a dependency
   of the overrides.

This way, when the problem gets fixed upstream, our override will fail
to build: the 'shouldFail x' dependency will fail, because 'x' will now
work. The "fix" for these failures is to throw away the overrides :)