Add support for relative requires

[Guldoman]

This keeps a stack of require "paths", and uses the topmost one when resolving relative requires to calculate the "absolute path".

From inside "path_1.path_2":

• Calling require ".b" will require "path_1.path_2.b".
• Calling require "..b" will require "path_1.b".

This has a few caveats:

1. The relative require must be called "inside" the parent require; this means that requires called "later" will have the wrong path.
   So:

   require ".b" -- OK
   
   local function blabla()
     require ".c" -- Not OK
   end

2. The path traversal is limited by the parent path; so if the parent path is core.plugins and the relative path is .......plugin_name, the resulting path is plugin_name.

(2) might be considered a feature, to limit the require range and not go outside our managed directories (even thought a plugin could still add its own custom search path).

To provide a workaround for (1), I added get_current_require_path which will return the topmost path in the stack. This still needs to be called inside the parent require, but its result can be used later.
So:

require ".b" -- OK
local path = get_current_require_path() -- OK

local function blabla()
  require ".c" -- Not OK
  require(path .. ".c") -- OK
  get_current_require_path() -- Not OK
end

Suggestions for a better name for get_current_require_path are welcome.
Should we also add a level parameter to get_current_require_path, so that path traversal doesn't need to be implemented by each plugin that uses it?

EDIT: If we want to avoid using pcall, we could use a to-be-closed object that does the stack popping.