Allow self-chats in nested task folders

[EricMichaelSmith]

Patch description
Currently, it doesn't seem like we can run a self-chat world if the value of --task has a '.' in it, because the code to specify a self-chat world is parsed after we return from load_world_module() in that case. This PR shifts that logic around to not fall back on the default world before checking if we want to do self-chats.

Testing steps
CI checks, as well as checking that the following command (referring to an internal task) is now possible due to this change:

parlai self_chat \
--num-self-chats 1 \
--outfile ~/__test.jsonl \
--selfchat-max-turns 7 \
--model_file zoo:blender/blender_90M/model \
--beam-block-full-context True \
--task parlai_internal.tasks.bias_dialogue.cities.agents

Also, added testing for the self-chat and interactive mode for a long-form task string

[stephenroller]

This seems reasonable? Any way we can add some tests? Maybe just refer to the public ones via absolute paths?

[stephenroller]

Wtf why did we lower it

[EricMichaelSmith]

Convention, probably? Namely, none of our task folder names (convai2, blended_skill_talk, etc.) are capitalized

[stephenroller]

Can we upgrade to f-strings while we're at it

[EricMichaelSmith]

Yes, changed