Requirements merge is broken

[JoshKarpel]

Because of this kind of logic:

base_requirements = base.pop('requirements', None)
settings_requirements = from_settings.pop('requirements', None)
if base_requirements is not None and settings_requirements is not None:
    core['requirements'] = f'({base_requirements}) && ({settings_requirements})'

which appears in two places in options.py, extra requirements are only merged in if there are also other requirements coming from some other place. This is obviously broken, because sometimes the other set of requirements won't exist. This needs better logic.