fix: Fix purgeOnStart on aliased storages

[janbuchar]

• closes Aliased storages survive purgeOnStart #3998

Unnamed storages (default and aliased) are now purged on start, named ones are not — crawlee-python's rule. The fs backend sweeps the storage directories, so leftovers from a previous process are caught too.

Two adjacent bugs, one commit each:

• createDatasetBackend() / ({}) didn't open the default storage, though StorageIdentifier says they do
• the __default__ sentinel leaked into the directory name, so default storages lived in storage/datasets/__default__ rather than default

ThrottlingRequestManager (#3741) sub-queues are alias-keyed, so they now only survive a restart with purgeOnStart off — as its docs already said.

[barjin]

Can we give this a more prominent place in the docs somewhere? Do I understand right that

const cheerioQueue = await RequestQueue.open({ alias: 'cheerio' });

const browserCrawler = new PlaywrightCrawler(
    // queue: default
    (enqueueLinks) => {
        enqueueLinks({queue: cheerioQueue})
    }
);
await browserCrawler.run([...]);

const cheerioCrawler = new CheerioCrawler(cheerioQueue);
await cheerioCrawler.run();

This will fail as the cheerioQueue gets purged on cheerioCrawler.run()? If so, we should imo have a separate docs example / guide on how to run multiple crawlers, as this is a fairly popular pattern afaik.

nvm, I see that calling purgeDefaultStorages from crawler.run() will call backend.purge() only once 👍

I suppose this is also different from the on-platform behaviour, right?

[janbuchar]

I suppose this is also different from the on-platform behaviour, right?

Purging isn't really a thing on Apify platform, you get fresh default storages at the start of each run and aliased storages need to be either created, or mapped to IDs of storages created according to the Actor output config.