Fix the debugger

[axelson]

The debugger was inadvertently broken in #93 which the change from the deprecated ([2]) Code.load_file/1 to Code.require_file/1. The compatibility docs for 1.10 [1] list both Code.require_file/1 and Code.compile_file/1 as possible replacements for Code.load_file/1. In our case we need Code.compile_file/1 instead of Code.require_file/1 since Code.require_file/1 tracks which files have been required and compiled and does not compile them more than once. Specifically the docs state:

If require_file/2 is called more than once with a given file, that file will be compiled only once

So since Code.load_file/1 would always compile the file we need to use Code.compile_file/1 rather than Code.require_file/1. When Code.require_file/1 was used, the call in
ElixirLS.Debugger.Server.change_env/1 would have no effect because the file was previously required. By changing to Code.compile_file/1 the project will always be reloaded by compiling it's mix.exs file.

[1] https://hexdocs.pm/elixir/1.10.2/compatibility-and-deprecations.html#table-of-deprecations
[2] elixir-lang/elixir#7201

Fixes #131

[lukaszsamson]

Nice find. It was quite hard to decide between those two as it was not clear what the original intention was.

[axelson]

Good point, added a couple code comments to explain the reasoning in-line.