fix(checkhealth): handle nested lua/ directory #32918

Problem: :checkhealth fails if plugin has nested "lua/" directory

Solution: trim `{runtimepath}/lua` from fullpath to get subpath
(`./**/{health, health/init.lua}`)

test/functional/fixtures/lua/test_plug/lua/health.lua

@@ -0,0 +1,8 @@
+local M = {}
+
+M.check = function()
+  vim.health.start('nested lua/ directory')
+  vim.health.ok('everything is ok')
+end
+
+return M