ycmd: add gopls completer
ycmd: add gopls completer

ycmd gives 'no completer support' because of missing 'gopls'.
Add this as a conditional dependency.

Refactor the 'mkdir -p' step to be conditional per-dependency,
and placed just before the link step.
This is mostly for legibility but also pedantic correctness:
do not create a directory unless it will be used.

Signed-off-by: Sirio Balmelli <50e55ca76dd9b5a7b391c1cd5393aefacbbd09c8@b-ad.ch>

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs