[WIP] Global runtime directory path resolution

[ebr]

Introduces a paths module which has no dependencies outside of stdlib, and so can be used outside of venv. Paths are defined as data structures and are dynamically updated. Intended usage: this module should be a single entrypoint to path resolution across the app and any scripts.

Also introduces a global runtime_dir or RD object, which uses the paths module and manages/validates the directory structure, including helpful messaging.

Fixes #1613, supercedes #1615. Related to #1786 - path resolution will need to be updated there to use the new paths module