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.

src/nvim/generators/gen_api_dispatch.lua

@@ -419,7 +419,7 @@ local function process_function(fn)
 
   if not fn.fast then
     write_shifted_output(output, string.format([[
-    if (!nlua_is_deferred_safe(lstate)) {
+    if (!nlua_is_deferred_safe()) {
       return luaL_error(lstate, e_luv_api_disabled, "%s");
     }
     ]], fn.name))