Fix catalog._datasets access for KedroDataCatalog

[ElenaKhaustova]

Description

Solves #4436

Development notes

After we implemented lazy dataset loading for KedroDataCatalog, we differed internal attributes _datasets and _lazy_datasets and started tracking them separately. At the same time, for old DataCatalog, we extensively used the internal _datasets attribute across the codebase, which caused a compatibility issue as the _datasets attribute stored a different set of datasets for old (all datasets) and new catalogs (all materialized datasets).

This behaviour didn't affect kedro run as we did a warm-up before the run, so needed datasets were materialized and there was no discrepancy.

We fixed that at the KedroDataCatalog level and left TODOs for when we switch to the new catalog completely:

1. Added __datasets property for KedroDataCatalog, so we could still differ materialized and lazy datasets at the catalog level
2. Made KedroDataCatalog.datatest return both materialized and lazy datasets
3. Updated __getattribute__ for KedroDataCatalog, so if someone calls KedroDataCatalog._datatest they get the same result as KedroDataCatalog.datatest. We could add just _datasets property but for that, we needed to modify CatalogProtocol which we preferred to avoid.
4. Added a test to check that when having materialized and lazy datasets both KedroDataCatalog.datatest and KedroDataCatalog._datatest return the same.

Note: These changes are temporary and needed for compatibility with the old catalog. They will be removed when migrating to the new catalog; corresponding TODOs were left.

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Checklist

• Read the contributing guidelines
• Signed off each commit with a Developer Certificate of Origin (DCO)
• Opened this PR as a 'Draft Pull Request' if it is work-in-progress
• Updated the documentation to reflect the code changes
• Added a description of this change in the RELEASE.md file
• Added tests to cover my changes
• Checked if this change will affect Kedro-Viz, and if so, communicated that with the Viz team

[merelcht]

Perhaps add a test to check that kedro catalog list now works?

Otherwise LGTM 👍

[ElenaKhaustova]

@merelcht

Perhaps add a test to check that kedro catalog list now works?

I think we might need to tackle this for all commands together. Now all catalog CLI commands unit tests only run on the old catalog.

I tried it, but because of the way catalog CLI tests are organized, I couldn't do it in a reasonable time. I think we need to create a separate ticket to address that, so this one doesn't block the release in case it takes time.

[merelcht]

I tried it, but because of the way catalog CLI tests are organized, I couldn't do it in a reasonable time. I think we need to create a separate ticket to address that, so this one doesn't block the release in case it takes time.

Sounds good. Can you create the ticket?

[ElenaKhaustova]

Sounds good. Can you create the ticket?

#4442

[lrcouto]

LGTM!