Add code action for hiding imports

[fendor]

Initially taken from: haskell/haskell-ide-engine#1191

It would be nice if there was a code action for hiding imports which are unused and being shadowed by local declarations.

This is complicated because it requires some detailed information on the provenance of these names in order to hide them (for example if they are record fields).
An example:

Given the following warning:

triangle/Main.hs:83:26: warning: [-Wname-shadowing]
    This binding for ‘window’ shadows the existing bindings
      imported from ‘Graphics.Vulkan’ at triangle/Main.hs:(29,1)-(45,59)
      (and originally defined in ‘Graphics.Vulkan.Extensions.VK_NN_vi_surface’)
      imported from ‘Graphics.Vulkan’ at triangle/Main.hs:(29,1)-(45,59)
      (and originally defined in ‘Graphics.Vulkan.Extensions.VK_KHR_xlib_surface’)
      imported from ‘Graphics.Vulkan’ at triangle/Main.hs:(29,1)-(45,59)
      (and originally defined in ‘Graphics.Vulkan.Extensions.VK_KHR_xcb_surface’)
      imported from ‘Graphics.Vulkan’ at triangle/Main.hs:(29,1)-(45,59)
      (and originally defined in ‘Graphics.Vulkan.Extensions.VK_KHR_android_surface’)

As a first step, we can try to implement a mechanism to hide a symbol that is name shadowing and can improve this with some time!

There are two different implementation paths ahead:

• Porting the Hsimport Plugin and adding suitable code-actions (might be hard since the hsimport plugin is a complicated piece of software)
• Modifying the import suggestion logic of ghcide: https://github.com/digital-asset/ghcide/blob/master/src/Development/IDE/Plugin/CodeAction.hs#L140

[pepeiborra]

I think this was closed by mistake, #1193 seems unrelated

[alanz]

HLS at 0fd73f8 gives really good code actions for resolving ambiguous functions

It would be great to see the same machinery modified for name shadowing cases. At the very least to hide the imported clashing name if it is not actually used in the file.