Add repo href to source facts cache key

[monoxgas]

I propose extending the __source cache key used for saving repo source facts in watchSource to include the .href of the associated anchor element.

This is useful for cases where we are overriding/updating the repo_url link between different pages on the same site (useful for mono/multi-repo scenarios). As it currently stands, the cache will always hit if any repo facts have ever been queried and gives no consideration for caching per-distinct repo - resulting in the return of facts for the first rendered Source component for all others.

I had also considered using _md_hash() on the .href to reduce noise in the cache key, but I think it's best that @squidfunk make any final calls on what/how to make the cache value distinct - open to suggestions.

Potential side-effects:

• Existing facts cached under the generic __source are not removed or modified
• The change would force a one-time re-query for facts in existing sites once updated

[squidfunk]

Thanks for the PR. In order for us to decide on whether this can be merged, could I ask you to please provide a minimal reproduction that showcases the problem you're aiming to solve? It's easier for us to analyze and understand the implications. Also, we can test the behavior before/after.