Internal functions can't be accessed in submodules

Given a `helper.vy` module used by a `main.vy` contract:

```vyper
# helper.vy

def foo():
   pass
```

```vyper
# main.vy

import helper

@external
def bar():
    helper.foo()
```

A `VyperContract` doesn't have a way to access the `foo` method without:
1. Using `.eval` (slow)
2. Using function injection (slow)
3. Deploying the module standalone (not always possible if `uses` is used)