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/testluaplugin/lua/testluaplugin/hello.lua

@@ -0,0 +1,7 @@
+local function hello()
+    return "hello from lua"
+end
+
+return {
+    hello = hello
+}

src/testdir/testluaplugin/lua/testluaplugin/init.lua

@@ -0,0 +1,5 @@
+local hello = require('testluaplugin/hello').hello
+
+return {
+    hello = hello
+}