Resolves the path of the module referred to by 'from ..x import y'.

def resolve_frompath(pkgpath, relpath, level=0):
    """Resolves the path of the module referred to by 'from ..x import y'."""
    if level == 0:
        return relpath
    parts = pkgpath.split('.') + ['_']
    parts = parts[:-level] + (relpath.split('.') if relpath else [])
    return '.'.join(parts)