Change METADATA.toml "dependencies" into internal and external

The context is I'm looking into #5768.

There's a lot of logic we have that either assumes or checks that entries in "requires" are typeshed stub packages. It seems potentially useful to split "requires" into "requires_typeshed" (a list of required packages that originate from typeshed) and "requires_external" (a list of external dependencies). Doing so could e.g. help avoid issues where we have typos in some requires_typeshed deps or potentially implicitly trust packages that start with "types-". Could also make it easier to build tooling (e.g. knowing what we could put on MYPYPATH to test against latest typeshed).