cabal v2-run File.hs should use runghc

[phadej]

... and have some flag to force compilation, if that's really needed.

For non-GHC compilers, the compilation code paths is then the only option.

This is one step to make cabal v2-run File.hs more responsive.

[phadej]

On the other though, it might be confusing when cabal run exe-in-project is first compiled. Maybe the default between compilation and interpretation should be consistent. Yet, adding runghc support is a good start.

[hvr]

This would be more a lot more convenient to do if we had ghc --run support (see https://gitlab.haskell.org/ghc/ghc/issues/18011) just like we don't invoke the repl via ghci but rather directly via ghc --interactive. Passing GHC arguments to runghc is a bit awkward and I'm not sure if runghc has any support for "response files".