defp deps do
    [
      {:decimal, "~> 2.1"},
    ]
end

% mix deps.get
** (Mix) No package with name decimal (from: mix.exs) in registry
% hex.search decimal
Package             Description                                            Version  URL                                         
decimal             Arbitrary precision decimal arithmetic.                2.2.0    https://hex.pm/packages/decimal             

% rm ~/.hex/cache.ets 
% mix deps.get
Resolving Hex dependencies...
Resolution completed in 0.008s
New:
  decimal 2.2.0