feat(lua): document support of packages with v:lua syntax

this already worked in 0.5 but was not properly documented or tested

runtime/doc/lua.txt

@@ -393,6 +393,14 @@ where the args are converted to Lua values. The expression >
 is equivalent to the Lua chunk >
     return somemod.func(...)
 
+In addition, functions of packages can be accessed like >
+    v:lua.require'mypack'.func(arg1, arg2)
+    v:lua.require'mypack.submod'.func(arg1, arg2)
+Note: only single quote form without parens is allowed. Using
+`require"mypack"` or `require('mypack')` as prefixes do NOT work (the latter
+is still valid as a function call of itself, in case require returns a useful
+value).
+
 The `v:lua` prefix may be used to call Lua functions as |method|s. For
 example: >
     arg1->v:lua.somemod.func(arg2)
@@ -409,7 +417,8 @@ For example consider the following Lua omnifunc handler: >
     end
     vim.api.nvim_buf_set_option(0, 'omnifunc', 'v:lua.mymod.omnifunc')
 
-Note: the module ("mymod" in the above example) must be a Lua global.
+Note: the module ("mymod" in the above example) must either be a Lua global,
+or use the require syntax as specified above to access it from a package.
 
 Note: `v:lua` without a call is not allowed in a Vimscript expression:
 |Funcref|s cannot represent Lua functions. The following are errors: >

test/functional/lua/luaeval_spec.lua

@@ -527,6 +527,12 @@ describe('v:lua', function()
     ]]}
   end)
 
+  it('supports packages', function()
+    command('set pp+=test/functional/fixtures')
+    eq('\tbadval', eval("v:lua.require'leftpad'('badval')"))
+    eq(9003, eval("v:lua.require'bar'.doit()"))
+  end)
+
   it('throw errors for invalid use', function()
     eq('Vim(let):E15: Invalid expression: v:lua.func', pcall_err(command, "let g:Func = v:lua.func"))
     eq('Vim(let):E15: Invalid expression: v:lua', pcall_err(command, "let g:Func = v:lua"))