don't use isdefined in stale_cachefile

`isdefined` will immediatly resolve the binding. Since we are only
trying to skip the path lookup the weaker version `isbindingresolved`
is enough. `isbindingresolved` is also used for this purpose in
`read_verify_mod_list` in `src/dump.c`.

Author: vchuravy

base/loading.jl

@@ -750,7 +750,7 @@ function stale_cachefile(modpath::String, cachefile::String)
             if mod == :Main || mod == :Core || mod == :Base
                 continue
             # Module is already loaded
-            elseif isdefined(Main, mod)
+            elseif isbindingresolved(Main, mod)
                 continue
             end
             name = string(mod)