Use cabal-add to fill missing build-depends automatically

[Bodigrim]

When HLS gets an error "module Bla.Bla.Bla is a member of a hidden package bla" it should suggest a quick fix to insert the dependency on bla into Cabal file using https://github.com/Bodigrim/cabal-add and carry on.

cabal-add draft API can be examined here: https://hackage.haskell.org/package/cabal-add-0.1/candidate/docs/Distribution-Client-Add.html. My impression is that HLS should be very much capable to create Config and write the result of executeConfig to the disk.

CC @VeryMilkyJoe on behalf of hls-cabal-plugin.

[georgefst]

This has been on the todo list for a long time: #155 (but maybe cabal-add is the solution!).

[Bodigrim]

cabal-add preserves the formatting of the original file. I tested it on complete set of Cabal files from Hackage.

[fendor]

Adding cabal-add should be a matter of parsing the error message, guessing the relevant .cabal file and then running cabal-add on it. #3778 can be used as an inspiration for this particular code action.
Nothing difficult about it, just someone needs to take the time to implement it!

[fendor]

Implemented by #4360