When it happens it wastes 2 seconds which is NOT included in the
normal busted timing info. It is hard to correct this, but we
can at least print a warning when this happens.
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.
Quoting the existing docs:
Packages are loaded. These are plugins, as above [&runtimepath], but
found in the "start" directory of each entry in 'packpath'. Every
plugin directory found is added in 'runtimepath' and then the plugins
are sourced.
Also tj didn't think I could do it.
