[v.10.4.x] Explore: lookup datasource by name when present in legacy URLs

[cwbriones]

Backport 867d36f from #85222

What is this feature?

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• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[ifrost]

LGTM 👍