For consideration: Bring main extract function into package

[carderne]

@Lkruitwagen @frandorr

Moving all the extract_patches logic into the main package under satextractor.extractor, leaving only the HTTP and BQ stuff in the Flask app.

Seems cleaner, and gives the option to do the extractions in-process if wanted! Could make it easier to try out sat-extractor on a local machine with a local output directory, no need to spin up PubSub, CloudRun etc.