Feature/auto require without init

[sewbacca]

This pull request solves #875. In completion, it respects now custom require paths.
Consider this file strucuture:

• mylib
  • init.lua with return { }
• test.lua with myli
  And the package path: ?.lua;?/init.lua.
  It will now allow auto require mylib or mylib.init.
  As for nested modules, it will try do match the most specific module first:
• mylib
  • inner
    • init.lua with return { }
  • init.lua with return { }
• test.lua with inn
inn will match to inner, but mylib won't point to mylib.inner but mylib.init.
  It didn't broke any tests and I tested it manually. I would have written tests, but I don't know how I can add additional package paths in crossfile/completion.lua.

[sumneko]

You can infer these tests: ./test/tclient/tests

[sewbacca]

Okay I've noticed I can just manipulate the config in place. To not bleed into other tests, I added a local function called WITH_CONFIG which essentially scopes the changes to the config for the given function.
I'm unsure if these tests suffice, but I think they cover all essential use cases (except the one module in one file ones - I think I already saw tests for those). See this commit for more details.

[sumneko]

Please resolve conflicts

[sewbacca]

Oh this explains why I was having issues lately, I was just 799 commits behind.
Its done now :D