fix(health): show error if setup didn't run

* Fix back bootstrapping and healthcheck for fresh install with no packages to fetch.

* Revert changes to bootstrapping, make checkhealth produce more meaningful message.

lua/lazy/health.lua

@@ -53,6 +53,9 @@ function M.check()
   end
 
   local spec = Config.spec
+  if spec == nil then
+    error("No plugins loaded. Did you forget to run `require(\"lazy\").setup()`?")
+  else
     for _, plugin in pairs(spec.plugins) do
       M.check_valid(plugin)
       M.check_override(plugin)
@@ -71,6 +74,7 @@ function M.check()
       end
     end
   end
+end
 
 ---@param plugin LazyPlugin
 function M.check_valid(plugin)