Ensure Main.eval and Main.include exist in embedded julia

[c42f]

These two functions need to be added to Main as part of jl_init(), but only when Base is present to avoid bootstrap issues.

Fixes #28825

Obviously __preinit_threads__ is a weirdly named function to be moving this to, but this part of the init stage seems to be appropriate (ie, right after jl_add_standard_imports(jl_main_module)). Renaming/cleanup suggestions welcome.

[JeffBezanson]

__preinit_threads__ is specifically for thread state, so I don't think it should go there. I think the best way to do it is in init.c right after the call to jl_add_standard_imports(jl_main_module), by calling jl_module_import.

[c42f]

Yes, I think that's better, I pushed an update.