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/api/keysets.lua

@@ -48,5 +48,8 @@ return {
     "style";
     "noautocmd";
   };
+  runtime = {
+    "is_lua";
+  };
 }