fix(runtime): don't use regexes inside lua require'mod'

Fixes #15147 and fixes #15497. Also sketch "subdir" caching. Currently
this only caches whether an rtp entry has a "lua/" subdir but we could
consider cache other subdirs potentially or even "lua/mybigplugin/"
possibly.

Note: the async_leftpad test doesn't actually fail on master, at least
not deterministically (even when disabling the fast_breakcheck
throttling). It's still useful as a regression test for further changes
and included as such.

test/functional/core/startup_spec.lua

@@ -328,6 +328,15 @@ describe('startup', function()
     eq({9003, '\thowdy'}, exec_lua [[ return { _G.y, _G.z } ]])
   end)
 
+  it("handles require from &packpath in an async handler", function()
+      -- NO! you cannot just speed things up by calling async functions during startup!
+      -- It doesn't make anything actually faster! NOOOO!
+    pack_clear [[ lua require'async_leftpad'('brrrr', 'async_res') ]]
+
+    -- haha, async leftpad go brrrrr
+    eq('\tbrrrr', exec_lua [[ return _G.async_res ]])
+  end)
+
   it("handles :packadd during startup", function()
     -- control group: opt/bonus is not availabe by default
     pack_clear [[

test/functional/fixtures/pack/foo/start/fancyplugin/after/lua/fancy_y.lua

@@ -0,0 +1 @@
+return "I am fancy_y.lua"

test/functional/fixtures/pack/foo/start/fancyplugin/after/lua/fancy_z.lua

@@ -0,0 +1 @@
+return "I am fancy_z.lua"

test/functional/fixtures/pack/foo/start/fancyplugin/lua/fancy_x.lua

@@ -0,0 +1 @@
+return "I am fancy_x.lua"

test/functional/fixtures/pack/foo/start/fancyplugin/lua/fancy_x/init.lua

@@ -0,0 +1 @@
+return "I am init.lua of fancy_x!"

test/functional/fixtures/pack/foo/start/fancyplugin/lua/fancy_y/init.lua

@@ -0,0 +1,2 @@
+
+return "I am init.lua of fancy_y!"

test/functional/fixtures/start/nvim-leftpad/lua/async_leftpad.lua

@@ -0,0 +1,3 @@
+return function (val, res)
+  vim.loop.new_async(function() _G[res] = require'leftpad'(val) end):send()
+end

test/functional/lua/vim_spec.lua

@@ -2255,7 +2255,7 @@ end)
 
 describe('lua: require("mod") from packages', function()
   before_each(function()
-    command('set rtp+=test/functional/fixtures')
+    command('set rtp+=test/functional/fixtures pp+=test/functional/fixtures')
   end)
 
   it('propagates syntax error', function()
@@ -2266,4 +2266,13 @@ describe('lua: require("mod") from packages', function()
 
     matches("unexpected symbol", syntax_error_msg)
   end)
+
+  it('uses the right order of mod.lua vs mod/init.lua', function()
+    -- lua/fancy_x.lua takes precedence over lua/fancy_x/init.lua
+    eq('I am fancy_x.lua', exec_lua [[ return require'fancy_x' ]])
+    -- but lua/fancy_y/init.lua takes precedence over after/lua/fancy_y.lua
+    eq('I am init.lua of fancy_y!', exec_lua [[ return require'fancy_y' ]])
+    -- safety check: after/lua/fancy_z.lua is still loaded
+    eq('I am fancy_z.lua', exec_lua [[ return require'fancy_z' ]])
+  end)
 end)