`typecheck` should ignore `.gitignore`-ed files

[adrien-piquerez-da]

Is your enhancement request related to a problem? Please describe.

Running haskell-language-server with no argument runs the typecheck command on all haskell files from the working directory recursively. In a repo using Bazel it can find Haskell files from the cache directory, which fails to typecheck.

Describe the solution you'd like

haskell-language-server should not look into the cache directories. To do so it could use .gitignore if it finds one.

Describe alternatives you've considered

An alternative is to specify all relevant sub-directories manually like this:

haskell-language-server typecheck a b c d

Additional context

A similar issue was originally reported in haskell/hie-bios#299

[fendor]

That sounds reasonable to me, PRs welcome!

Changes should be quite isolated in https://github.com/haskell/haskell-language-server/blob/master/ghcide/src/Development/IDE/Main.hs#L385