Remove unconventional location for header files

[robertoaloi]

This PR is an alternative approach to #49.

The rationale for this solution is that moving the file, instead of aliasing it, is a much simpler approach and it makes the code base use a more conventional inclusion pattern. While having sub-directories within include and including headers with `-include('my_dir/my_header.hrl') technically works in OTP, it's a non conventional choice that makes it harder to include the file from external applications. Tools such as Erlang LS may require additional tweaking by the user to get code navigation and other features working with the code base, too.

Breaking backward compatibility is probably not a big deal in this case, given that the header file was explicitly marked as internal in the previous implementation. Making the move explicit would also allow users to immediately spot (and correct) the difference at compile time and take the opportunity to cleanup extra paths and directories in their configuration which were only present due to the current setup. So, it feels like a step in the right direction.

[robertoaloi]

I opened this PR immediately before realizing you already have #49 as an alternative solution. Let me check if that works for our use case...

[robertoaloi]

@dumbbell Any opinion on this?

[coveralls]

Coverage remained the same at 77.564% when pulling 9a9f7a2 on erlang-ls:avoid-internal-includes into 39981b9 on yakaz:master.

[dumbbell]

I agree with you, the breakage should be ok.