Remove haskell-src-exts dependency from the core package

[thomasjm]

Probably want to either a) figure out how to use TH to do main function autodetection or b) come up with a more ad-hoc heuristic. A less desirable solution would be to move some test discovery stuff into its own package.

[Bodigrim]

Note that sandwich depends on haskell-src-exts not only directly, but also transitively, e. g., via string-interpolate.

[thomasjm]

Oh good point, I hadn't noticed that.