Refactor dynamic country package imports to instead directly reference country packages

[anth-volk]

As proposed by @ldorner, we should refactor akin to the below:

import policyengine_uk
import policyengine_us
...

class PolicyEngineCountry:
    COUNTRY_MODULES = {
         'uk': policyengine_uk,
         'us': policyengine_us,
         ...
     }

    def __init__(self, country_package_name: str, country_id: str):
        self.country_package_name = country_package_name
        self.country_id = country_id
        self.country_package = self.COUNTRY_MODULES[country_package_name]
        ...