[Bug] Sandbox importer not respecting relative levels for pass through

[cretz]

Describe the bug

Some relative imports are respected, but ones that call __import__ with a level > 0 are not properly. This surfaced during a non-binary install of Pydantic for #207. We need to mimic https://github.com/python/cpython/blob/c8c6113398ee9a7867fe9b08bc539cceb61e2aaa/Lib/importlib/_bootstrap.py#L1255-L1258.

[cretz]

Here is the helper needed to build the fully qualified name:

def _resolve_module_name(name: str, globals: Optional[Mapping[str, object]], level: int) -> str:
    if level == 0:
        return name
    # Calc the package from globals
    package = _calc___package__(globals or {})
    # Logic taken from importlib._resolve_name
    bits = package.rsplit(".", level - 1)
    if len(bits) < level:
        raise ImportError("Attempted relative import beyond top-level package")
    base = bits[0]
    return f"{base}.{name}" if name else base

# Copied from importlib._calc__package__
def _calc___package__(globals: Mapping[str, object]) -> str:
    """Calculate what __package__ should be.
    __package__ is not guaranteed to be defined or could be set to None
    to represent that its proper value is unknown.
    """
    package = globals.get("__package__")
    spec = globals.get("__spec__")
    if package is not None:
        if spec is not None and package != spec.parent:
            warnings.warn("__package__ != __spec__.parent "
                           f"({package!r} != {spec.parent!r})",
                           DeprecationWarning, stacklevel=3)
        return package
    elif spec is not None:
        return spec.parent
    else:
        warnings.warn("can't resolve package from __spec__ or __package__, "
                       "falling back on __name__ and __path__",
                       ImportWarning, stacklevel=3)
        package = globals["__name__"]
        if "__path__" not in globals:
            package = package.rpartition('.')[0]
    return package