bug[ux]: interface files allow initializes/uses statements

[charles-cooper]

          we can import stateful modules to the `vyi` files and eg call `initializes` on them:

# ifile.vyi
import test

initializes: test

@external
def foo(a: test.Foo) -> test.Foo:
    ...

# test.vy

c: uint256

flag Foo:
    Blah

@external
def foo() -> Foo:
    return Foo.Blah

works eg with -f annotated_ast

Originally posted by @cyberthirst in #4290 (comment)