Pass package dependency information to Fourmolu

[georgefst]

HLS 1.8 finally picks up Fourmolu > 0.7. In fact it ships with 0.8 on all GHC versions.

Unfortunately this causes a regression in recognising operator fixities for packages whose fixities were previously hardcoded. At least in environments with no Cabal file. I hit this with SBV, within seconds of installing the new HLS version.

This current implementation feels like a total hack. I could do with suggestions.

TODO

• get fixities from env directly, rather than even having to read cabal file? this may be the key to doing this robustly
• implement this for Ormolu
• wrap version CPP around cfgDependencies, putting it in Shim.hs
• check compatiblity across GHC API versions - given this involves units, I'd expect the API used to have changed in 9.4
• regression test
• maybe combine noCabal and packages in to a single option, seeing as they're closely related and introduced in the same version of Fourmolu