[proposal] make a warning when trying to use Capital word in module attributes like "@A" or support it

[forbitc...@gmail.com]

When compile this:

```

defmoudle M do

  @A

  IO.inspect @A

end

```

result is:

```

:ok

warning: module attribute @__aliases__ was set but never used

```

It's because of the AST of `@A` is

```

iex(1)> quote do 

...(1)> @A

...(1)> end

{:@, [context: Elixir, import: Kernel],

 [{:__aliases__, [context: Elixir, alias: false], [:A]}]}

```

and the AST of `@a 1` is

```

iex(2)> quote do

...(2)> @a 1

...(2)> end

{:@, [context: Elixir, import: Kernel], [{:a, [context: Elixir], [1]}]}

```

So, the compiler just read `@A` as "@__aliases__ :A". 

Current warning message is hard to reason about. Maybe we can have a better warning message, or support the capital module attributes.

[José Valim]

Yes, please open up an issue.

--

José Valim

www.plataformatec.com.br

Skype: jv.ptec

Founder and Director of R&D