prosody, prosodyctl: Load loader directly from source directory This should ensure that loader.lua is loaded from the source directory and does the right thing when installed with `make install` There are currently three ways Prosody can be run: - Directly from the source directory, like `./prosody` - Installed with `make install` - Installed into Lua paths with e.g. dh-lua or luarocks In the first two cases, Lua search paths need to include the source directory and ensure that `require "prosody.util.json"` ends up loading `util/json.lua` relative to the installation (CFG_SOURCEDIR) or the source checkout. Finally, in the last case where Prosody resources are installed under the 'prosody.*' namespace in regular Lua search paths, then loader.lua should activate the compatibility mode that makes sure that both `require"util.json"` and `require"prosody.util.json"` both resolve to `(one path from of package.path)/prosody/util/json.lua`