patch 8.2.0858: not easy to require Lua modules

Problem:    Not easy to require Lua modules.
Solution:   Improve use of Lua path. (Prabir Shrestha, closes #6098)

src/testdir/test_lua.vim

@@ -536,6 +536,11 @@ func Test_lua_open()
   %bwipe!
 endfunc
 
+func Test_update_package_paths()
+  set runtimepath+=./testluaplugin
+  call assert_equal("hello from lua", luaeval("require('testluaplugin').hello()"))
+endfunc
+
 " Test vim.line()
 func Test_lua_line()
   new