[loader] Pass ALC to mono_domain_loaded_internal, and then to domain search hook...

[loader] Pass ALC to mono_domain_loaded_internal, and then to domain search hook (mono/mono#15743)

* Add a temporary mono_domain_ambient_alc function

Just use this to mark places where we need to pass through an ALC

* [runtime] Mark mono_assembly_loaded_full external only.

Runtime should use mono_assembly_loaded_internal.

* [loader] Add mono_install_assembly_search_hook_v2

Use it to install mono_domain_assembly_search and
mono_domain_assembly_postload_search.

The principal difference is that we can pass an assembly load context to the
hook.  But also we cleanup the "requesting assembly" hack for
mono_domain_assembly_postload_search.

* [domain] Comments and mono_assembly_get_alc ()

* [debugger] Comment on suspicious loader lock

Commit migrated from https://github.com/mono/mono/commit/a37ad98b470381950c876bb0928cab57fe8156b2