In a project with lots of constraints it's not always obvious from
which file a constraint is coming from. Also we are currently showing
a big `TODO` message in place of the file name which is not nice.

Prior to this patch

```
$ cabal build all
Resolving dependencies...
cabal: Could not resolve dependencies:
[__0] trying: some-package-0.24.1.2 (user goal)
[__1] trying: foldl-1.4.8 (dependency of some-package)
[__2] next goal: mwc-random (dependency of foldl)
[__2] rejecting: mwc-random-0.15.0.0 (constraint from project config TODO
requires ==0.14.0.0)
[__2] trying: mwc-random-0.14.0.0
[__3] next goal: math-functions (dependency of mwc-random)
[__3] rejecting: math-functions-0.3.4.0, math-functions-0.3.3.0,
math-functions-0.3.2.1, math-functions-0.3.2.0, math-functions-0.3.1.0,
math-functions-0.3.0.2, math-functions-0.3.0.1, math-functions-0.3.0.0,
math-functions-0.2.1.0, math-functions-0.2.0.2, math-functions-0.2.0.1,
math-functions-0.2.0.0, math-functions-0.1.7.0, math-functions-0.1.6.0,
math-functions-0.1.5.2, math-functions-0.1.5.1, math-functions-0.1.4.0,
math-functions-0.1.3.0, math-functions-0.1.1.2, math-functions-0.1.1.1,
math-functions-0.1.1.0, math-functions-0.1.0.0 (constraint from project config
TODO requires ==0.3.4.1)
[__3] fail (backjumping, conflict set: math-functions, mwc-random)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: mwc-random, math-functions, foldl,
some-package
Try running with --minimize-conflict-set to improve the error message.
```

Note the TODO. No idea where th constraint is coming from :(

With this patch applied:

```
Resolving dependencies...
cabal: Could not resolve dependencies:
[__0] trying: some-package-0.24.1.2 (user goal)
[__1] trying: foldl-1.4.8 (dependency of some-packagge)
[__2] next goal: mwc-random (dependency of foldl)
[__2] rejecting: mwc-random-0.15.0.0 (constraint from project config
/Users/a602232/git/haskell-libs/cabal.project.freeze requires ==0.14.0.0)
[__2] trying: mwc-random-0.14.0.0
[__3] next goal: math-functions (dependency of mwc-random)
[__3] rejecting: math-functions-0.3.4.0, math-functions-0.3.3.0,
math-functions-0.3.2.1, math-functions-0.3.2.0, math-functions-0.3.1.0,
math-functions-0.3.0.2, math-functions-0.3.0.1, math-functions-0.3.0.0,
math-functions-0.2.1.0, math-functions-0.2.0.2, math-functions-0.2.0.1,
math-functions-0.2.0.0, math-functions-0.1.7.0, math-functions-0.1.6.0,
math-functions-0.1.5.2, math-functions-0.1.5.1, math-functions-0.1.4.0,
math-functions-0.1.3.0, math-functions-0.1.1.2, math-functions-0.1.1.1,
math-functions-0.1.1.0, math-functions-0.1.0.0 (constraint from project config
/Users/a602232/git/haskell-libs/cabal.project.freeze requires ==0.3.4.1)
[__3] fail (backjumping, conflict set: math-functions, mwc-random)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: mwc-random, math-functions, foldl,
some-package
Try running with --minimize-conflict-set to improve the error message.
```

Note that we now explicitly say where the constraint is coming from!