Add method to dump repo dbs to JSON files

Add `dump_db_to_json_files()` which allows for a repository database
file to be read and all of its members to be serialized as JSON and
written to files based upon the packages respective pkgbases.
For the JSON serialization orjson is used, as it provides the highest
speeds available and seems the most correct implementation.

Add a fixture to create and destroy a temporary directory (used to write
JSON files to).
Add tests for `dump_db_to_json_files()`.
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