make vim.loop == require'luv'

This avoids initializing libluv a second time if a plugin invokes
require'luv'. It is probably not an issue, but better to be safe.

test/functional/lua/loop_spec.lua

@@ -142,4 +142,8 @@ describe('vim.loop', function()
     ]])
     eq({blocking=false, mode='n'}, exec_lua("return _G.mode"))
   end)
+
+  it("is equal to require('luv')", function()
+    eq(true, exec_lua("return vim.loop == require('luv')"))
+  end)
 end)